Bitmask requires wasm_simd_post_mvp because it was merged into SIMD
proposal after 84 cut, which Origin Trial starts.

For now, bitmask is the only instruction that requires this flag to be
set, and no other post mvp instructions are included in the fuzzer.

We should revert this change (and also move bitmask out of the flag)
after this OT is over.

Bug: chromium:1098666
Change-Id: I7d45c805aaa18bfc1a5180e70b912d5f17d4a31d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2264628Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68549}