Fix LoadFieldByIndex to take mutable heap-numbers into account.

src/arm64/lithium-arm64.cc

Index: src/arm64/lithium-arm64.cc
diff --git a/src/arm64/lithium-arm64.cc b/src/arm64/lithium-arm64.cc
index 60bf51ebbdc7d33170faff92dcf106d777a1f7c4..9ec6e47698e2887a803228e9dcaa52a45bfe9e71 100644
--- a/src/arm64/lithium-arm64.cc
+++ b/src/arm64/lithium-arm64.cc
@@ -2561,7 +2561,9 @@ LInstruction* LChunkBuilder::DoCheckMapValue(HCheckMapValue* instr) {
 LInstruction* LChunkBuilder::DoLoadFieldByIndex(HLoadFieldByIndex* instr) {
   LOperand* object = UseRegisterAtStart(instr->object());
   LOperand* index = UseRegister(instr->index());
-  return DefineAsRegister(new(zone()) LLoadFieldByIndex(object, index));
+  LLoadFieldByIndex* load = new(zone()) LLoadFieldByIndex(object, index);
+  LInstruction* result = DefineSameAsFirst(load);
+  return AssignPointerMap(result);
 }