CopyPropagation for SILValues with ownership.

This is a simple "utility" pass that canonicalizes SSA SILValues with
respect to copies and destroys. It is a self-contained, provably
complete pass that eliminates spurious copy_value instructions from
scalar SSA SILValues. It fundamentally depends on ownership SIL, but
otherwise can be run efficiently after any other pass. It separates
the pure problem of handling scalar SSA values from the more important
and complex problems:

- Promoting variables to SSA form (PredictableMemOps and Mem2Reg
  partially do this).

- Optimizing copies within "SIL borrow" scopes (another mandatory pass
  will be introduced to do this).

- Composing and decomposing aggregates (SROA handles some of this).

- Coalescing phis (A BlockArgumentOptimizer will be introduced as part
  of AddressLowering).

- Removing unnecessary retain/release when nothing within its scope
  may release the same object (ARC Code Motion does some of this).

Note that removing SSA copies was more obviously necessary before the
migration to +0 argument convention.

Author: atrick

include/swift/SILOptimizer/PassManager/Passes.def

@@ -108,6 +108,8 @@ PASS(ConditionForwarding, "condition-forwarding",
      "Conditional Branch Forwarding to Fold SIL switch_enum")
 PASS(CopyForwarding, "copy-forwarding",
      "Copy Forwarding to Remove Redundant Copies")
+PASS(CopyPropagation, "copy-propagation",
+     "Copy propagation to Remove Redundant SSA Copies")
 PASS(EpilogueARCMatcherDumper, "sil-epilogue-arc-dumper",
      "Print Epilogue retains of Returned Values and Argument releases")
 PASS(EpilogueRetainReleaseMatcherDumper, "sil-epilogue-retain-release-dumper",

lib/SILOptimizer/Transforms/CMakeLists.txt

@@ -9,6 +9,7 @@ silopt_register_sources(
   CSE.cpp
   ConditionForwarding.cpp
   CopyForwarding.cpp
+  CopyPropagation.cpp
   DeadCodeElimination.cpp
   DeadObjectElimination.cpp
   DeadStoreElimination.cpp