Implement these 4 instructions for arm64 and arm Liftoff:
- i64x2.widen_low_i32x4_s
- i64x2.widen_high_i32x4_s
- i64x2.widen_low_i32x4_u
- i64x2.widen_high_i32x4_u

Drive-by cleanup of the test case to make it clearer that we are
checking against an unsigned result.

Bug: v8:10972
Change-Id: I509a8df8a6f2109417ad5aaaa0324ced50bdc84a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2626713Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72074}