[stubs] Use CSA::IsHeapNumberMap() instead of manual map comparing.

src/builtins/builtins-conversion.cc

Index: src/builtins/builtins-conversion.cc
diff --git a/src/builtins/builtins-conversion.cc b/src/builtins/builtins-conversion.cc
index 995f1185465111336834e6859c767aa322ea74a7..3a24c98d32b5f84dc4227609eb910743d6501253 100644
--- a/src/builtins/builtins-conversion.cc
+++ b/src/builtins/builtins-conversion.cc
@@ -184,9 +184,8 @@ void Builtins::Generate_ToString(compiler::CodeAssemblerState* state) {
 
   assembler.Bind(&not_string);
   {
-    assembler.GotoUnless(
-        assembler.WordEqual(input_map, assembler.HeapNumberMapConstant()),
-        &not_heap_number);
+    assembler.GotoUnless(assembler.IsHeapNumberMap(input_map),
+                         &not_heap_number);
     assembler.Goto(&is_number);
   }