This prototypes i32x4.widen_i8x16_s and i32x4.widen_i8x16_u for the
interpreter.

This is the first instruction of its kind, a post-mvp, unary operation
that takes one immediate. Which is why there are more changes to the
decoder than usual.

Bug: v8:11297
Change-Id: Ib5c58965e0cba8d7a395b0dc57673110bc60e87c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2617385Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72170}