Ruby: Avoid computing full fastTC for AstNode::getParent

DIL before
```
                                                   /* AST::AstNode */ AST#87953007::Cached::TAstNode result) =
  fastTC(AST#a6718388::AstNode::getAChild#0#dispred#ff/2)
.

Completion#445d5844::mayRaise#1#f(/* Call::Call */ unique AST#87953007::Cached::TAstNode c)
:-
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode call_result#2 |
    exists(/* ControlFlowGraphImpl::Trees::BodyStmtTree */ AST#87953007::Cached::TAstNode bst |
      (
        (
          project#Expr#6fb2af19::BodyStmt::getRescue#1#dispred#fff(bst),
          ControlFlowGraphImpl#288ae92e::Trees::BodyStmtTree#class#f(bst)
        );
        (
          exists(/* Expr::StmtSequence */ dontcare AST#87953007::Cached::TAstNode _ |
            Expr#6fb2af19::BodyStmt::getEnsure#0#dispred#ff(bst, _)
          ),
          ControlFlowGraphImpl#288ae92e::Trees::BodyStmtTree#class#f(bst)
        )
      ),
      ControlFlowGraphImpl#288ae92e::Trees::BodyStmtTree#class#f(bst),
      project#ControlFlowGraphImpl#288ae92e::Trees::StmtSequenceTree::getBodyChild#2#dispred#ffff(bst,
                                                                                                  call_result#2)
    ),
    (
      (c = call_result#2, Call#841c84e8::Call#f(c));
      (
        #AST#a6718388::AstNode::getAChild#0#dispredPlus#ff(call_result#2, c),
        Call#841c84e8::Call#f(c)
      )
    )
  )
.
```

DIL after
```
incremental
Completion#445d5844::getARescuableBodyChild#0#f(/* AST::AstNode */ unique AST#87953007::Cached::TAstNode result)
:-
  exists(/* ControlFlowGraphImpl::Trees::BodyStmtTree */ AST#87953007::Cached::TAstNode bst |
    (
      (
        exists(dontcare int _,
               /* Expr::RescueClause */ dontcare AST#87953007::Cached::TAstNode _1 |
          Expr#6fb2af19::BodyStmt::getRescue#1#dispred#fff(bst, _, _1)
        ),
        ControlFlowGraphImpl#288ae92e::Trees::BodyStmtTree#class#f(bst)
      );
      (
        exists(/* Expr::StmtSequence */ dontcare AST#87953007::Cached::TAstNode _ |
          Expr#6fb2af19::BodyStmt::getEnsure#0#dispred#ff(bst, _)
        ),
        ControlFlowGraphImpl#288ae92e::Trees::BodyStmtTree#class#f(bst)
      )
    ),
    ControlFlowGraphImpl#288ae92e::Trees::BodyStmtTree#class#f(bst),
    exists(boolean arg2, dontcare int _ |
      arg2 = true,
      ControlFlowGraphImpl#288ae92e::Trees::StmtSequenceTree::getBodyChild#2#dispred#ffff(bst,
                                                                                          _,
                                                                                          arg2,
                                                                                          result)
    )
  );
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode call_result#5 |
    rec Completion#445d5844::getARescuableBodyChild#0#f(call_result#5),
    exists(cached dontcare string _ |
      AST#a6718388::AstNode::getAChild#1#dispred(call_result#5, _, result)
    )
  )
| [base_case]
  exists(/* ControlFlowGraphImpl::Trees::BodyStmtTree */ AST#87953007::Cached::TAstNode bst |
    (
      (
        project#Expr#6fb2af19::BodyStmt::getRescue#1#dispred#fff(bst),
        ControlFlowGraphImpl#288ae92e::Trees::BodyStmtTree#class#f(bst)
      );
      (
        exists(/* Expr::StmtSequence */ dontcare AST#87953007::Cached::TAstNode _ |
          Expr#6fb2af19::BodyStmt::getEnsure#0#dispred#ff(bst, _)
        ),
        ControlFlowGraphImpl#288ae92e::Trees::BodyStmtTree#class#f(bst)
      )
    ),
    ControlFlowGraphImpl#288ae92e::Trees::BodyStmtTree#class#f(bst),
    project#ControlFlowGraphImpl#288ae92e::Trees::StmtSequenceTree::getBodyChild#2#dispred#ffff(bst,
                                                                                                result)
  )
| [delta_order]
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode call_result#5 |
    delta previous rec Completion#445d5844::getARescuableBodyChild#0#f(call_result#5),
    project#AST#a6718388::AstNode::getAChild#1#dispred(call_result#5, result)
  ),
  not(previous rec Completion#445d5844::getARescuableBodyChild#0#f(result))
.
```

Author: hvitved

ruby/ql/lib/codeql/ruby/controlflow/internal/Completion.qll

@@ -71,19 +71,23 @@ private predicate completionIsValidForStmt(AstNode n, Completion c) {
   c = TReturnCompletion()
 }
 
-/**
- * Holds if `c` happens in an exception-aware context, that is, it may be
- * `rescue`d or `ensure`d. In such cases, we assume that the target of `c`
- * may raise an exception (in addition to evaluating normally).
- */
-private predicate mayRaise(Call c) {
-  exists(Trees::BodyStmtTree bst | c = bst.getBodyChild(_, true).getAChild*() |
+private AstNode getARescuableBodyChild() {
+  exists(Trees::BodyStmtTree bst | result = bst.getBodyChild(_, true) |
     exists(bst.getARescue())
     or
     exists(bst.getEnsure())
   )
+  or
+  result = getARescuableBodyChild().getAChild()
 }
 
+/**
+ * Holds if `c` happens in an exception-aware context, that is, it may be
+ * `rescue`d or `ensure`d. In such cases, we assume that the target of `c`
+ * may raise an exception (in addition to evaluating normally).
+ */
+private predicate mayRaise(Call c) { c = getARescuableBodyChild() }
+
 /** A completion of a statement or an expression. */
 abstract class Completion extends TCompletion {
   private predicate isValidForSpecific(AstNode n) {