Subzero: Allow delaying Phi lowering until after register allocation.

src/IceOperand.h

Index: src/IceOperand.h
diff --git a/src/IceOperand.h b/src/IceOperand.h
index 6d31127166e303e45ac23415eb6882a569cb1f6e..76a01810bf1bb6700a7147fdf327e1bbb5eacd09 100644
--- a/src/IceOperand.h
+++ b/src/IceOperand.h
@@ -398,6 +398,8 @@ public:
   void setIgnoreLiveness() { IgnoreLiveness = true; }
   bool getIgnoreLiveness() const { return IgnoreLiveness; }
 
+  bool needsStackSlot() const { return NeedsStackSlot; }
+  void setNeedsStackSlot() { NeedsStackSlot = true; }
   int32_t getStackOffset() const { return StackOffset; }
   void setStackOffset(int32_t Offset) { StackOffset = Offset; }
 
@@ -474,9 +476,9 @@ public:
 protected:
   Variable(OperandKind K, Type Ty, SizeT Index, const IceString &Name)
       : Operand(K, Ty), Number(Index), Name(Name), IsArgument(false),
-        IsImplicitArgument(false), IgnoreLiveness(false), StackOffset(0),
-        RegNum(NoRegister), RegNumTmp(NoRegister), Weight(1), LoVar(NULL),
-        HiVar(NULL) {
+        IsImplicitArgument(false), IgnoreLiveness(false), NeedsStackSlot(false),
+        StackOffset(0), RegNum(NoRegister), RegNumTmp(NoRegister), Weight(1),
+        LoVar(NULL), HiVar(NULL) {
     Vars = VarsReal;
     Vars[0] = this;
     NumVars = 1;
@@ -492,6 +494,9 @@ protected:
   // constructing and validating live ranges.  This is usually
   // reserved for the stack pointer.
   bool IgnoreLiveness;
+  // NeedsStackSlot starts out false, and is set to true once we know
+  // for sure that the variable needs a stack slot.
+  bool NeedsStackSlot;
   // StackOffset is the canonical location on stack (only if
   // RegNum==NoRegister || IsArgument).
   int32_t StackOffset;
@@ -578,6 +583,9 @@ public:
   // Initialize the state by traversing all instructions/variables in
   // the CFG.
   void init(MetadataKind TrackingKind);
+  // Add a single node.  This is called by init(), and can be called
+  // incrementally from elsewhere, e.g. after edge-splitting.
+  void addNode(CfgNode *Node);
   // Returns whether the given Variable is tracked in this object.  It
   // should only return false if changes were made to the CFG after
   // running init(), in which case the state is stale and the results