[turbofan] Support virtual register redirection in instruction selector.

src/compiler/instruction.h

Index: src/compiler/instruction.h
diff --git a/src/compiler/instruction.h b/src/compiler/instruction.h
index b2d0454db65cbc7ced77d0f57617c4aee34a7741..0c570f2b737998009076d477d5a7e18e8a49266c 100644
--- a/src/compiler/instruction.h
+++ b/src/compiler/instruction.h
@@ -1184,6 +1184,7 @@ class PhiInstruction final : public ZoneObject {
   PhiInstruction(Zone* zone, int virtual_register, size_t input_count);
 
   void SetInput(size_t offset, int virtual_register);
+  void RenameInput(size_t offset, int virtual_register);
 
   int virtual_register() const { return virtual_register_; }
   const IntVector& operands() const { return operands_; }
@@ -1251,6 +1252,7 @@ class InstructionBlock final : public ZoneObject {
 
   typedef ZoneVector<PhiInstruction*> PhiInstructions;
   const PhiInstructions& phis() const { return phis_; }
+  PhiInstruction* PhiAt(size_t i) const { return phis_[i]; }
   void AddPhi(PhiInstruction* phi) { phis_.push_back(phi); }
 
   void set_ao_number(RpoNumber ao_number) { ao_number_ = ao_number; }