[x64] Compare map instead of value to heap number map in ToStringStub.

Fixes a typo introduced earlier, where we compare the value to heap
number map instead of the map loaded previously.

TBR=jarin@chromium.org

Review URL: https://codereview.chromium.org/1355253002

Cr-Commit-Position: refs/heads/master@{#30859}

src/x64/code-stubs-x64.cc

@@ -3132,7 +3132,7 @@ void ToStringStub::Generate(MacroAssembler* masm) {
   __ bind(&not_string);
 
   Label not_heap_number;
-  __ CompareRoot(rax, Heap::kHeapNumberMapRootIndex);
+  __ CompareRoot(rdi, Heap::kHeapNumberMapRootIndex);
   __ j(not_equal, &not_heap_number, Label::kNear);
   __ bind(&is_number);
   NumberToStringStub stub(isolate());