Prototype these 6 instructions on arm64:

- 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

Drive-by fix:

- f64x2.promote_low_f32x4 accesses out of bounds for the global, the
result only has 2 doubles
- fcvtn in simulator needs to clear top bits of the Q reg

Bug: v8:11265
Change-Id: Icfb3338942f0d0374448fdcfef3847a6e3ce8ff6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2644066Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72373}