When converting a Smi to a String, we can skip the check for a
cached array index on the result in case of a number-to-string
cache hit. When trying to convert a String back to an index, the
inlined fast path can check for a cached index (in addition to
checking for a cached known negative).
Locally this yields about 5% on the JSTests/Proxies/GetIndex* tests.

Bug: chromium:1028021
Change-Id: I117eae01b1ad9c5d107ad7e598464b96dae9a6b9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1943160
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65299}