MIPS: Do not use the 64-bit floor and truncate instructions in fp32 mode.

These instructions have unpredictable result when the processor is in fp32 mode.

BUG=

TEST=mjsunit/math-floor-global,math-floor-local,math-floor-part1

Review URL: https://codereview.chromium.org/1116073002

Cr-Commit-Position: refs/heads/master@{#28194}

src/compiler/mips/instruction-selector-mips.cc

@@ -980,7 +980,8 @@ InstructionSelector::SupportedMachineOperatorFlags() {
   MachineOperatorBuilder::Flags flags =
       MachineOperatorBuilder::kFloat32Abs | MachineOperatorBuilder::kFloat64Abs;
 
-  if (IsMipsArchVariant(kMips32r2) || IsMipsArchVariant(kMips32r6)) {
+  if ((IsMipsArchVariant(kMips32r2) || IsMipsArchVariant(kMips32r6)) &&
+      IsFp64Mode()) {
     flags |= MachineOperatorBuilder::kFloat64RoundDown |
              MachineOperatorBuilder::kFloat64RoundTruncate;
   }