"t.Is(Type::Unique())" is more conservative and future-proof than
"!t.Maybe(Type::NumericOrString)".

Change-Id: I7d08244802feeb062fd2f8a9d8f3af85eb43bba3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2106207
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66768}