Subzero: Cleanly implement register allocation after phi lowering.

src/IceTargetLowering.cpp

Index: src/IceTargetLowering.cpp
diff --git a/src/IceTargetLowering.cpp b/src/IceTargetLowering.cpp
index 671bab029d0d30871a55e94f9a27052f27f36a9d..327fb411c9665cf06f5934e5d7f9f9e17b8fa05d 100644
--- a/src/IceTargetLowering.cpp
+++ b/src/IceTargetLowering.cpp
@@ -211,6 +211,17 @@ void TargetLowering::lower() {
   Context.advanceNext();
 }
 
+void TargetLowering::lowerInst(CfgNode *Node, InstList::iterator Next,
+                               InstHighLevel *Instr) {
+  Context.init(Node);
+  Context.setNext(Next);
+  Context.insert(Instr);
+  --Next;
+  assert(&*Next == Instr);
+  Context.setCur(Next);
+  lower();
+}
+
 void TargetLowering::lowerOther(const Inst *Instr) {
   (void)Instr;
   Func->setError("Can't lower unsupported instruction type");