Add lowering for I64x2 in S128Const and converting Int64x2 to
Int32x2.

Bug: v8:10507
Change-Id: I5bc40ae135fa00e31e901337b1a315f6ead14b02
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2410800Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69960}