Prototype these 6 instructions on arm:

- 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

For all these instructions we rely on having Q registers that map to S
registers, which means we can only use q0 to q7. We fix the src/dst
to q0 arbitrarily.

Bug: v8:11265
Change-Id: Ied95f2dde9859a60fc216ed67615f80e9d795bb7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2679842Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72567}