These flags are later packed into an InstructionCode by the
instruction selector, but not all bits are preserved. This CL adds a
few assertions and helper methods to clarify and make this a bit
safer.

Bug: v8:8888
Change-Id: I01adc9f7887cc21c623d790dc204ac1d4b9bc7ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2388116
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69728}