Subzero: Refactor x86 register definitions to use the alias mechanism.

src/IceOperand.cpp

 //===- subzero/src/IceOperand.cpp - High-level operand implementation -----===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 113 matching lines @@
   while (TrimmedBegin != Range.end() && TrimmedBegin->second <= Lower)
     ++TrimmedBegin;
 }
 
 IceString Variable::getName(const Cfg *Func) const {
   if (Func && NameIndex >= 0)
     return Func->getIdentifierName(NameIndex);
   return "__" + std::to_string(getIndex());
 }
 
-Variable *Variable::asType(Type Ty) {
+const Variable *Variable::asType(Type Ty, int32_t NewRegNum) const {
   // Note: This returns a Variable, even if the "this" object is a subclass of
   // Variable.
   if (!BuildDefs::dump() || getType() == Ty)
     return this;
   Variable *V = new (getCurrentCfgAllocator()->Allocate<Variable>())
       Variable(kVariable, Ty, Number);
   V->NameIndex = NameIndex;
-  V->RegNum = RegNum;
+  V->RegNum = NewRegNum == NoRegister ? RegNum : NewRegNum;
   V->StackOffset = StackOffset;
   return V;
 }
 
 RegWeight Variable::getWeight(const Cfg *Func) const {
   VariablesMetadata *VMetadata = Func->getVMetadata();
   return mustHaveReg() ? RegWeight(RegWeight::Inf)
                        : mustNotHaveReg() ? RegWeight(RegWeight::Zero)
                                           : VMetadata->getUseWeight(this);
 }
@@ skipping 393 matching lines @@
   if (getType() != IceType_i32 && getType() != IceType_i16 &&
       getType() != IceType_i8)
     return false;
   // The Following checks if the signed representation of Value is between
   // -Threshold/2 and +Threshold/2
   bool largerThanThreshold = Threshold / 2 + Value >= Threshold;
   return largerThanThreshold;
 }
 
 } // end of namespace Ice