Lithium: avoid registers for constants when possible.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index fbb7c289797d43ab3fee6320a201ff93cf72b160..0e75c1c22302ff9dd3dd2bbed3d357f8ae3d26e5 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -5122,7 +5122,6 @@ void LCodeGen::DoAllocate(LAllocate* instr) {
 
 
 void LCodeGen::DoDeferredAllocate(LAllocate* instr) {
-  Register size = ToRegister(instr->size());
   Register result = ToRegister(instr->result());
 
   // TODO(3095996): Get rid of this. For now, we need to make the
@@ -5131,8 +5130,16 @@ void LCodeGen::DoDeferredAllocate(LAllocate* instr) {
   __ Set(result, 0);
 
   PushSafepointRegistersScope scope(this);
-  __ Integer32ToSmi(size, size);
-  __ push(size);
+  if (instr->size()->IsRegister()) {
+    Register size = ToRegister(instr->size());
+    ASSERT(!size.is(result));
+    __ Integer32ToSmi(size, size);
+    __ push(size);
+  } else {
+    int32_t size = ToInteger32(LConstantOperand::cast(instr->size()));
+    __ Push(Smi::FromInt(size));
+  }
+
   if (instr->hydrogen()->CanAllocateInOldPointerSpace()) {
     CallRuntimeFromDeferred(
         Runtime::kAllocateInOldPointerSpace, 1, instr);