Subzero: Allow delaying Phi lowering until after register allocation.

src/IceTargetLowering.cpp

Index: src/IceTargetLowering.cpp
diff --git a/src/IceTargetLowering.cpp b/src/IceTargetLowering.cpp
index b18999f67ebb6e7013bd5cf3153bf4121f79e938..350605bb4758298a62a85b6c198ee0768110a140 100644
--- a/src/IceTargetLowering.cpp
+++ b/src/IceTargetLowering.cpp
@@ -44,12 +44,16 @@ cl::opt<int> NopProbabilityAsPercentage(
 
 void LoweringContext::init(CfgNode *N) {
   Node = N;
+  End = getNode()->getInsts().end();
+  rewind();
+  advanceForward(Next);
+}
+
+void LoweringContext::rewind() {
   Begin = getNode()->getInsts().begin();
   Cur = Begin;
-  End = getNode()->getInsts().end();
   skipDeleted(Cur);
   Next = Cur;
-  advanceForward(Next);
 }
 
 void LoweringContext::insert(Inst *Inst) {