MIPS: Use neg instruction in r6

src/crankshaft/mips/lithium-codegen-mips.cc

Index: src/crankshaft/mips/lithium-codegen-mips.cc
diff --git a/src/crankshaft/mips/lithium-codegen-mips.cc b/src/crankshaft/mips/lithium-codegen-mips.cc
index 2453ede9ff3eb1f492bd1c87732cf446edd19214..2d45919708fe2101d83bd1d104effbac86070ae7 100644
--- a/src/crankshaft/mips/lithium-codegen-mips.cc
+++ b/src/crankshaft/mips/lithium-codegen-mips.cc
@@ -3442,7 +3442,9 @@ void LCodeGen::DoMathPowHalf(LMathPowHalf* instr) {
   // Math.sqrt(-Infinity) == NaN
   Label done;
   __ Move(temp, static_cast<double>(-V8_INFINITY));
+  // Set up Infinity.
   __ Neg_d(result, temp);
+  // result is overwritten if the branch is not taken.
   __ BranchF(&done, NULL, eq, temp, input);
 
   // Add +0 to convert -0 to +0.