Prototype these 6 instructions on x64:

- f64x2.convert_low_i32x4_s
- f64x2.convert_low_i32x4_u
- i32x4.trunc_sat_f64x2_s_zero
- i32x4.trunc_sat_f64x2_u_zero
- f32x4.demote_f64x2_zero
- f64x2.promote_low_f32x4

Some of these code sequences make use of special masks, we keep them in
external references.

Code sequence based on suggestions at:
https://github.com/WebAssembly/simd/pull/383

Bug: v8:11265
Change-Id: Ied67d7b5b6beaaccac7c179ec13504482cb9c915
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2643562Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72297}