PPC: [turbofan] Implemented the TruncateFloat32ToUint64 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/1480243003

Cr-Commit-Position: refs/heads/master@{#32434}

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

@@ -938,7 +938,7 @@ void InstructionSelector::VisitTruncateFloat64ToInt64(Node* node) {
 
 
 void InstructionSelector::VisitTruncateFloat32ToUint64(Node* node) {
-  UNIMPLEMENTED();
+  VisitRR(this, kPPC_DoubleToUint64, node);
 }