- I32x4SConvertI16x8Low and I16x8SConvertI8x16Low becomes kArm64Sxtl
- I32x4SConvertI16x8High and I16x8SConvertI8x16High becomes kArm64Sxtl2
- I32x4UConvertI16x8Low and I16x8UConvertI8x16Low becomes kArm64Uxtl
- I32x4UConvertI16x8High and I16x8UConvertI8x16High becomes kArm64Uxtl2

This saves us 4 arch opcodes, and also later when we implement the i64x2
versions, we don't need to add any new opcodes.

Bug: v8:10930, v8:10972
Change-Id: I01f3ea78f0bc7de9026316379d9eefa18df3a0d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2441367
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70357}