Fix object initialization when slack tracking for it's map is still enabled.

src/crankshaft/arm64/lithium-codegen-arm64.cc

Index: src/crankshaft/arm64/lithium-codegen-arm64.cc
diff --git a/src/crankshaft/arm64/lithium-codegen-arm64.cc b/src/crankshaft/arm64/lithium-codegen-arm64.cc
index 5c90beb68ef79d5b81d0625e5073b6941ba1baf9..61413192b7a1ec03a16e8e08a0a35b47f8752577 100644
--- a/src/crankshaft/arm64/lithium-codegen-arm64.cc
+++ b/src/crankshaft/arm64/lithium-codegen-arm64.cc
@@ -1555,20 +1555,20 @@ void LCodeGen::DoAllocate(LAllocate* instr) {
   __ Bind(deferred->exit());
 
   if (instr->hydrogen()->MustPrefillWithFiller()) {
-    Register filler_count = temp1;
-    Register filler = temp2;
-    Register untagged_result = ToRegister(instr->temp3());
+    Register start = temp1;
+    Register end = temp2;
+    Register filler = ToRegister(instr->temp3());
+
+    __ Sub(start, result, kHeapObjectTag);
 
     if (instr->size()->IsConstantOperand()) {
       int32_t size = ToInteger32(LConstantOperand::cast(instr->size()));
-      __ Mov(filler_count, size / kPointerSize);
+      __ Add(end, start, size);
     } else {
-      __ Lsr(filler_count.W(), ToRegister32(instr->size()), kPointerSizeLog2);
+      __ Add(end, start, ToRegister(instr->size()));
     }
-
-    __ Sub(untagged_result, result, kHeapObjectTag);
-    __ Mov(filler, Operand(isolate()->factory()->one_pointer_filler_map()));
-    __ FillFields(untagged_result, filler_count, filler);
+    __ LoadRoot(filler, Heap::kOnePointerFillerMapRootIndex);
+    __ InitializeFieldsWithFiller(start, end, filler);
   } else {
     DCHECK(instr->temp3() == NULL);
   }