Add lowering for F64x2 in S128Const and converting to and from f64x2.

Bug: v8:10507
Change-Id: Ic2c4f1f41d3dd804e012a943391a46b534864b51
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2424679Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70097}