A64: Add and use a double register which holds the 0.0 value.

src/a64/lithium-codegen-a64.cc

Index: src/a64/lithium-codegen-a64.cc
diff --git a/src/a64/lithium-codegen-a64.cc b/src/a64/lithium-codegen-a64.cc
index 8e443b9fb086bc82bec5593c1ddd79e87fddff5c..5a7abda66275437d57a81c572957373c54682431 100644
--- a/src/a64/lithium-codegen-a64.cc
+++ b/src/a64/lithium-codegen-a64.cc
@@ -3917,9 +3917,7 @@ void LCodeGen::DoMathPowHalf(LMathPowHalf* instr) {
   __ B(&done, eq);
 
   // Add +0.0 to convert -0.0 to +0.0.
-  // TODO(jbramley): A constant zero register would be helpful here.
-  __ Fmov(double_scratch(), 0.0);
-  __ Fadd(double_scratch(), input, double_scratch());
+  __ Fadd(double_scratch(), input, fp_zero);
   __ Fsqrt(result, double_scratch());
 
   __ Bind(&done);