[ShrinkBorrowScope] Adopt BackwardReachability.

Replaced ShrinkBorrowScope's own data flow with the general
BackwardReachability.

Took this opportunity to refactor and document the utility.

Taken together these changes make ShrinkBorrowScope serve as a template
for a future LexicalDestroyHoisting which will operate on owned lexical
values (rather than guaranteed as here) and hoist destroy_values (rather
than end_borrows as here) but should otherwise be quite similar.

Author: nate-chandler

include/swift/SILOptimizer/Utils/CanonicalizeBorrowScope.h

@@ -148,7 +148,7 @@ class CanonicalizeBorrowScope {
 };
 
 bool shrinkBorrowScope(
-    BeginBorrowInst *bbi, InstructionDeleter &deleter,
+    BeginBorrowInst const &bbi, InstructionDeleter &deleter,
     SmallVectorImpl<CopyValueInst *> &modifiedCopyValueInsts);
 
 bool foldDestroysOfCopiedLexicalBorrow(BeginBorrowInst *bbi,

lib/SILOptimizer/Transforms/CopyPropagation.cpp

@@ -451,7 +451,7 @@ void CopyPropagation::run() {
     // at least once and then until each stops making changes.
     while (true) {
       SmallVector<CopyValueInst *, 4> modifiedCopyValueInsts;
-      auto shrunk = shrinkBorrowScope(bbi, deleter, modifiedCopyValueInsts);
+      auto shrunk = shrinkBorrowScope(*bbi, deleter, modifiedCopyValueInsts);
       for (auto *cvi : modifiedCopyValueInsts)
         defWorklist.updateForCopy(cvi);
       changed |= shrunk;