[compiler] Fix typing of NumberToString operator.

It must be monotone.

R=bmeurer@chromium.org

Bug: v8:7354
Change-Id: I08dcd3333518029eef08c074c2b91b5c20ad699e
Reviewed-on: https://chromium-review.googlesource.com/880982
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50801}

src/compiler/operation-typer.cc

@@ -507,7 +507,7 @@ Type* OperationTyper::NumberToInt32(Type* type) {
 
 Type* OperationTyper::NumberToString(Type* type) {
   DCHECK(type->Is(Type::Number()));
-
+  if (type->IsNone()) return type;
   if (type->Is(Type::NaN())) return singleton_NaN_string_;
   if (type->Is(cache_.kZeroOrMinusZero)) return singleton_zero_string_;
   return Type::SeqString();