This relands commit d04b5e47.

The fix here is in the assembler for pmovmskb, emit_optional_rex_32 should be
called after emitting the prefix byte.

Original change's description:
> [wasm-simd][liftoff][ia32][x64] Implement bitmask
>
> Implements i8x16 i16x8 i32x4 bitmask.
>
> This was merged into the proposal in
> https://github.com/WebAssembly/simd/pull/201/.
>
> Bug: v8:9909,v8:10308
> Change-Id: I882f0c2697213cdf593e745112e0897cee252009
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2222607
> Commit-Queue: Zhi An Ng <zhin@chromium.org>
> Reviewed-by: Clemens Backes <clemensb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#68090}

Bug: v8:9909, v8:10308
Change-Id: I4897585c86b87f72dc8f142b275171276d135a24
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2225090Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68106}