Port 3b6eb335

Original Commit Message:

    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.

R=zhin@chromium.org, joransiu@ca.ibm.com, junyan@redhat.com, midawson@redhat.com
BUG=
LOG=N

Bug: v8:11265
Change-Id: Ibef0b05fe1066cd79ddcda12787441b09cc8ede2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2684360Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#72600}