Implement these 6 instructions:

- 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

The code sequences are exactly the same as on x64.

Needed to add some more instructions, and we don't have macro lists for
these instructions yet, so individually define them for now. We can
factor them into lists in a future change.

Bug: v8:11265
Change-Id: I606e1226201e3c5ecdc7e3f611315437e917d77c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2668913Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72535}