MIPS: Optimize NumberTagU

Port r18162 (8377a09b)

Original commit message:
Optimize register constaints for NumberTagU Lithium instruction, and adjust code generated to match.

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/97083002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18165 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/lithium-codegen-mips.cc

@@ -4708,13 +4708,12 @@ void LCodeGen::DoNumberTagU(LNumberTagU* instr) {
     LNumberTagU* instr_;
   };
 
-  LOperand* input = instr->value();
-  ASSERT(input->IsRegister() && input->Equals(instr->result()));
-  Register reg = ToRegister(input);
+  Register input = ToRegister(instr->value());
+  Register result = ToRegister(instr->result());
 
   DeferredNumberTagU* deferred = new(zone()) DeferredNumberTagU(this, instr);
-  __ Branch(deferred->entry(), hi, reg, Operand(Smi::kMaxValue));
-  __ SmiTag(reg, reg);
+  __ Branch(deferred->entry(), hi, input, Operand(Smi::kMaxValue));
+  __ SmiTag(result, input);
   __ bind(deferred->exit());
 }
 

src/mips/lithium-mips.cc

@@ -1940,7 +1940,7 @@ LInstruction* LChunkBuilder::DoChange(HChange* instr) {
       LOperand* value = UseRegisterAtStart(val);
       if (val->CheckFlag(HInstruction::kUint32)) {
         LNumberTagU* result = new(zone()) LNumberTagU(value);
-        return AssignEnvironment(AssignPointerMap(DefineSameAsFirst(result)));
+        return AssignEnvironment(AssignPointerMap(DefineAsRegister(result)));
       } else if (val->HasRange() && val->range()->IsInSmiRange()) {
         return DefineAsRegister(new(zone()) LSmiTag(value));
       } else {