i8 and i16 nodes are stored in word32 nodes, when splat-ing them, we
need to make sure to handle overflow values and also sign extend them
correctly.

This fix allows us to pass simd-splat.js. It still fails on ARM
(non-simulator) due to a use of f32x4.min in the test.

Bug: v8:10507
Change-Id: I1507637a7edb33a530c84c85ee8d4acb481293e2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2430170Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70146}