X87: [TurboFan] enable the Float64RoundDown and Float64RoundTruncate operators in x87.

  X87 port already implemented Float64RoundDown and Float64RoundTruncate operators, not enabled yet.
  This CL would enable them.

BUG=

Review URL: https://codereview.chromium.org/1486483003

Cr-Commit-Position: refs/heads/master@{#32442}

src/compiler/x87/code-generator-x87.cc

@@ -1091,11 +1091,8 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
     case kX87Float64Round: {
       RoundingMode mode =
           static_cast<RoundingMode>(MiscField::decode(instr->opcode()));
-      if (mode == MiscField::encode(kRoundDown)) {
-        __ X87SetRC(0x0400);
-      } else {
-        __ X87SetRC(0x0c00);
-      }
+      // Set the correct round mode in x87 control register
+      __ X87SetRC((mode << 10));
 
       if (!instr->InputAt(0)->IsDoubleRegister()) {
         InstructionOperand* input = instr->InputAt(0);

src/compiler/x87/instruction-selector-x87.cc

@@ -1300,6 +1300,9 @@ InstructionSelector::SupportedMachineOperatorFlags() {
   if (CpuFeatures::IsSupported(POPCNT)) {
     flags |= MachineOperatorBuilder::kWord32Popcnt;
   }
+
+  flags |= MachineOperatorBuilder::kFloat64RoundDown |
+           MachineOperatorBuilder::kFloat64RoundTruncate;
   return flags;
 }