PPC: [turbofan] Implemented the TruncateFloat32ToInt64 TurboFan 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/1490513004

Cr-Commit-Position: refs/heads/master@{#32428}

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

@@ -928,7 +928,7 @@ void InstructionSelector::VisitChangeFloat64ToUint32(Node* node) {
 
 #if V8_TARGET_ARCH_PPC64
 void InstructionSelector::VisitTruncateFloat32ToInt64(Node* node) {
-  UNIMPLEMENTED();
+  VisitRR(this, kPPC_DoubleToInt64, node);
 }