Add a SSADestroyHoisting utility and pass

Extract and rewrite the destroy hoisting algorithm originally from
CopyForwarding (in 2014).

This is now a light-weight utility for hoisting destroy_addr
instructions. Shrinking an object's memory lifetime can allow removal
of copy_addr and other optimization.

This is extremely low-overhead and can run at any optimization level
without dependency on any analysis.

This algorithm is:
- Incremental
- SSA-based
- Canonical
- Free from alias analysis

See file-level comments.

The immediate purpose is to specify and test the constraints
introduced by adding lexical variable lifetimes to SIL semantics. It
can be used as a template for end_borrow hoisting.

Ultimately, this utility can be invoked within any pass that needs to
optimize a particular uniquely identified address. It will be used to
remove much of the complexity from CopyForwarding.

Author: atrick

include/swift/SILOptimizer/PassManager/Passes.def

@@ -194,6 +194,8 @@ PASS(DefiniteInitialization, "definite-init",
      "Definite Initialization for Diagnostics")
 PASS(DestroyHoisting, "destroy-hoisting",
      "Hoisting of value destroys")
+PASS(SSADestroyHoisting, "ssa-destroy-hoisting",
+     "Hoist destroy_addr for uniquely identified values")
 PASS(Devirtualizer, "devirtualizer",
      "Indirect Call Devirtualization")
 PASS(DiagnoseInfiniteRecursion, "diagnose-infinite-recursion",

lib/SILOptimizer/Transforms/CMakeLists.txt

@@ -35,6 +35,7 @@ target_sources(swiftSILOptimizer PRIVATE
   SILLowerAggregateInstrs.cpp
   SILMem2Reg.cpp
   SILSROA.cpp
+  SSADestroyHoisting.cpp
   SimplifyCFG.cpp
   Sink.cpp
   SpeculativeDevirtualizer.cpp