Subzero: Minor refactoring/additions in preparation for phi edge splitting.

The only functional change (though not actually visible at this point) is that redundant assignment elimination is moved into a separate pass.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/672393003

src/IceTargetLoweringX8632.h

diff --git a/src/IceTargetLoweringX8632.h b/src/IceTargetLoweringX8632.h
index 20d7150..88b75ac 100644
--- a/src/IceTargetLoweringX8632.h
+++ b/src/IceTargetLoweringX8632.h
@@ -35,7 +35,7 @@
   void translateO2() override;
   bool doBranchOpt(Inst *I, const CfgNode *NextNode) override;
 
-  Variable *getPhysicalRegister(SizeT RegNum) override;
+  Variable *getPhysicalRegister(SizeT RegNum, Type Ty = IceType_void) override;
   IceString getRegName(SizeT RegNum, Type Ty) const override;
   llvm::SmallBitVector getRegisterSet(RegSetMask Include,
                                       RegSetMask Exclude) const override;