[CopyPropagation] Added lexical destroy folding.

The new utility folds patterns like

  TOP:
    // %borrowee is some owned value
    %lifetime = begin_borrow %borrowee

  BOTTOM:
    // %copy is some transitive copy of %borrowee
    apply %f(%copy)
    end_borrow %lifetime
    destroy_value %borrowee

into

  TOP:
    %move = move_value [lexical] %borrowee
    %lifetime begin_borrow [lexical] %move
  BOTTOM:
    end_borrow %lifetime
    apply %f(%move)

It is intended to be run after ShrinkBorrowScope moves the end_borrow up
to just before a relevant apply and after CanonicalizeOSSALifetime moves
destroy_value instructions up to just after their last guaranteed use,
at which point these patterns will exist.

Author: nate-chandler

include/swift/SILOptimizer/Utils/CanonicalizeBorrowScope.h

@@ -151,6 +151,10 @@ bool shrinkBorrowScope(
     BeginBorrowInst *bbi, InstructionDeleter &deleter,
     SmallVectorImpl<CopyValueInst *> &modifiedCopyValueInsts);
 
+bool foldDestroysOfCopiedLexicalBorrow(BeginBorrowInst *bbi,
+                                       DominanceInfo &dominanceTree,
+                                       InstructionDeleter &deleter);
+
 } // namespace swift
 
 #endif // SWIFT_SILOPTIMIZER_UTILS_CANONICALIZEBORROWSCOPES_H

lib/SILOptimizer/Utils/CMakeLists.txt

@@ -18,6 +18,7 @@ target_sources(swiftSILOptimizer PRIVATE
   InstructionDeleter.cpp
   InstOptUtils.cpp
   KeyPathProjector.cpp
+  LexicalDestroyFolding.cpp
   LoadStoreOptUtils.cpp
   LoopUtils.cpp
   OptimizerStatsUtils.cpp