[PeepholeOptimizer] Look through PHIs to find additional register sources

Reapply 243271 with more fixes; although we are not handling multiple
sources with coalescable copies, we were not properly skipping this
case.

- Teaches the ValueTracker in the PeepholeOptimizer to look through PHI
instructions.
- Add findNextSourceAndRewritePHI method to lookup into multiple sources
returnted by the ValueTracker and rewrite PHIs with new sources.

With these changes we can find more register sources and rewrite more
copies to allow coaslescing of bitcast instructions. Hence, we eliminate
unnecessary VR64 <-> GR64 copies in x86, but it could be extended to
other archs by marking "isBitcast" on target specific instructions. The
x86 example follows:

A:
  psllq %mm1, %mm0
  movd  %mm0, %r9
  jmp C

B:
  por %mm1, %mm0
  movd  %mm0, %r9
  jmp C

C:
  movd  %r9, %mm0
  pshufw  $238, %mm0, %mm0

Becomes:

A:
  psllq %mm1, %mm0
  jmp C

B:
  por %mm1, %mm0
  jmp C

C:
  pshufw  $238, %mm0, %mm0

Differential Revision: http://reviews.llvm.org/D11197
rdar://problem/20404526

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243486 91177308-0d34-0410-b5e6-96231b3b80d8

Author: bcardosolopes

include/llvm/Target/TargetInstrInfo.h

@@ -1266,6 +1266,33 @@ class TargetInstrInfo : public MCInstrInfo {
   unsigned CallFrameSetupOpcode, CallFrameDestroyOpcode;
 };
 
+/// \brief Provide DenseMapInfo for TargetInstrInfo::RegSubRegPair.
+template<>
+struct DenseMapInfo<TargetInstrInfo::RegSubRegPair> {
+  typedef DenseMapInfo<unsigned> RegInfo;
+
+  static inline TargetInstrInfo::RegSubRegPair getEmptyKey() {
+    return TargetInstrInfo::RegSubRegPair(RegInfo::getEmptyKey(),
+                         RegInfo::getEmptyKey());
+  }
+  static inline TargetInstrInfo::RegSubRegPair getTombstoneKey() {
+    return TargetInstrInfo::RegSubRegPair(RegInfo::getTombstoneKey(),
+                         RegInfo::getTombstoneKey());
+  }
+  /// \brief Reuse getHashValue implementation from
+  /// std::pair<unsigned, unsigned>.
+  static unsigned getHashValue(const TargetInstrInfo::RegSubRegPair &Val) {
+    std::pair<unsigned, unsigned> PairVal =
+        std::make_pair(Val.Reg, Val.SubReg);
+    return DenseMapInfo<std::pair<unsigned, unsigned>>::getHashValue(PairVal);
+  }
+  static bool isEqual(const TargetInstrInfo::RegSubRegPair &LHS,
+                      const TargetInstrInfo::RegSubRegPair &RHS) {
+    return RegInfo::isEqual(LHS.Reg, RHS.Reg) &&
+           RegInfo::isEqual(LHS.SubReg, RHS.SubReg);
+  }
+};
+
 } // End llvm namespace
 
 #endif