A64: Implement and use FillFields

src/a64/lithium-a64.cc

Index: src/a64/lithium-a64.cc
diff --git a/src/a64/lithium-a64.cc b/src/a64/lithium-a64.cc
index 8307882a5bbddf3f6b2284e32da282d786c92c1b..d44e2107fc0ecaa62d5443693fdd24c89a84f572 100644
--- a/src/a64/lithium-a64.cc
+++ b/src/a64/lithium-a64.cc
@@ -878,7 +878,8 @@ LInstruction* LChunkBuilder::DoAllocate(HAllocate* instr) {
   LOperand* size = UseRegisterOrConstant(instr->size());
   LOperand* temp1 = TempRegister();
   LOperand* temp2 = TempRegister();
-  LAllocate* result = new(zone()) LAllocate(context, size, temp1, temp2);
+  LOperand* temp3 = instr->MustPrefillWithFiller() ? TempRegister() : NULL;
+  LAllocate* result = new(zone()) LAllocate(context, size, temp1, temp2, temp3);
   return AssignPointerMap(DefineAsRegister(result));
 }