Motivation:
In the wasm-gc proposal, structs and arrays are allowed to store
elements of packed types i8 and i16.

Changes:
- Add i8 and i16 to ValueType.
- Fix all case switches to handle the new cases.
- Add a couple helper methods to ValueType and improve the
  implementation/usage of a couple more.

Bug: v8:7748
Change-Id: I527cfe5acf5d877fc38e4212174ba9f9de5c40ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2215046Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67994}