In preparation for adding new NCI (and TP) code kinds.

- Free the unused bit in Code::flags.
- Be more precise about the flag field sizes (int32 instead of int).
- Add and refactor related static asserts.

Bug: v8:8888
Change-Id: Ice0d4df9de528de77dfb5c04279cfdc4b030efc9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2328788
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69213}