Subzero: Add a peephole to fuse cmpxchg w/ later cmp+branch.

src/IceTargetLowering.cpp

Index: src/IceTargetLowering.cpp
diff --git a/src/IceTargetLowering.cpp b/src/IceTargetLowering.cpp
index 05ae1216547b4f6d7fe2d30d5f0d1ba1cb0d3555..2a9b8c431ac6fbd42cabba5f8bfb9384017829bb 100644
--- a/src/IceTargetLowering.cpp
+++ b/src/IceTargetLowering.cpp
@@ -38,12 +38,12 @@ void LoweringContext::insert(Inst *Inst) {
   Inst->updateVars(getNode());
 }
 
-void LoweringContext::skipDeleted(InstList::iterator &I) {
+void LoweringContext::skipDeleted(InstList::iterator &I) const {
   while (I != End && (*I)->isDeleted())
     ++I;
 }
 
-void LoweringContext::advance(InstList::iterator &I) {
+void LoweringContext::advance(InstList::iterator &I) const {
   if (I != End) {
     ++I;
     skipDeleted(I);