Optimizer: remove the CopyForwarding pass

This pass removes `copy_addr` instructions.
However, it has some problems which causes compiler crashes.
It's not worth fixing these bugs because
1. Most copy_addrs can be eliminated by TempRValueElimination and TempLValueElimination.
2. Once we have opaque value we don't need copy_addr elimination, anyway.

rdar://162212460

Author: eeckstein

include/swift/SILOptimizer/PassManager/Passes.def

@@ -254,8 +254,6 @@ LEGACY_PASS(ConstantEvaluatorTester, "test-constant-evaluator",
       "Test constant evaluator")
 LEGACY_PASS(ConstantEvaluableSubsetChecker, "test-constant-evaluable-subset",
      "Test Swift code snippets expected to be constant evaluable")
-LEGACY_PASS(CopyForwarding, "copy-forwarding",
-     "Copy Forwarding to Remove Redundant Copies")
 LEGACY_PASS(CopyPropagation, "copy-propagation",
      "Copy propagation to Remove Redundant SSA Copies, pruning debug info")
 LEGACY_PASS(MandatoryCopyPropagation, "mandatory-copy-propagation",

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -459,12 +459,6 @@ void addFunctionPasses(SILPassPipelinePlan &P,
     P.addDestroyAddrHoisting();
   }
 
-  // Propagate copies through stack locations.  Should run after
-  // box-to-stack promotion since it is limited to propagating through
-  // stack locations. Should run before aggregate lowering since that
-  // splits up copy_addr.
-  P.addCopyForwarding();
-
   // This DCE pass is the only DCE on ownership SIL. It can cleanup OSSA related
   // dead code, e.g. left behind by the ObjCBridgingOptimization.
   P.addDCE();

lib/SILOptimizer/Transforms/CMakeLists.txt

@@ -9,7 +9,6 @@ target_sources(swiftSILOptimizer PRIVATE
   COWOpts.cpp
   CSE.cpp
   ConditionForwarding.cpp
-  CopyForwarding.cpp
   CopyPropagation.cpp
   DeadCodeElimination.cpp
   DeadObjectElimination.cpp