Subzero: Allow delaying Phi lowering until after register allocation.

src/IceTargetLowering.h

Index: src/IceTargetLowering.h
diff --git a/src/IceTargetLowering.h b/src/IceTargetLowering.h
index e46399d311e6aa73ffc7d8e658c772df3b194b4a..7aa24832a0089fae0f6d099564388f3055395307 100644
--- a/src/IceTargetLowering.h
+++ b/src/IceTargetLowering.h
@@ -64,6 +64,7 @@ public:
   Inst *getLastInserted() const;
   void advanceCur() { Cur = Next; }
   void advanceNext() { advanceForward(Next); }
+  void rewind();
   void setInsertPoint(const InstList::iterator &Position) { Next = Position; }
 
 private:
@@ -134,8 +135,18 @@ public:
   void doAddressOpt();
   // Randomly insert NOPs.
   void doNopInsertion();
-  // Lowers a single instruction.
+  // Lowers a single non-Phi instruction.
   void lower();
+  // Does preliminary lowering of the set of Phi instructions in the
+  // current node.  The main intention is to do what's needed to keep
+  // the unlowered Phi instructions consistent with the lowered
+  // non-Phi instructions, e.g. to lower 64-bit operands on a 32-bit
+  // target.
+  virtual void prelowerPhis() {}
+  // Lowers a list of "parallel" assignment instructions representing
+  // a topological sort of the Phi instructions.
+  virtual void lowerPhiAssignments(CfgNode *Node,
+                                   const AssignList &Assignments) = 0;
   // Tries to do branch optimization on a single instruction.  Returns
   // true if some optimization was done.
   virtual bool doBranchOpt(Inst * /*I*/, const CfgNode * /*NextNode*/) {