PPC: [turbofan] Implemented the optional Float32RoundTruncate operator.

R=ahaas@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1480213003

Cr-Commit-Position: refs/heads/master@{#32427}

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

@@ -1159,7 +1159,7 @@ void InstructionSelector::VisitFloat64RoundUp(Node* node) {
 
 
 void InstructionSelector::VisitFloat32RoundTruncate(Node* node) {
-  UNREACHABLE();
+  VisitRR(this, kPPC_TruncateDouble, node);
 }
 
 
@@ -1691,6 +1691,7 @@ InstructionSelector::SupportedMachineOperatorFlags() {
          MachineOperatorBuilder::kFloat64RoundDown |
          MachineOperatorBuilder::kFloat32RoundUp |
          MachineOperatorBuilder::kFloat64RoundUp |
+         MachineOperatorBuilder::kFloat32RoundTruncate |
          MachineOperatorBuilder::kFloat64RoundTruncate |
          MachineOperatorBuilder::kFloat64RoundTiesAway |
          MachineOperatorBuilder::kWord32Popcnt |