Implement conversion of an i8x16 node to a f32x4 node.

Bug: v8:10507
Change-Id: Ifefffb779dbf25b57eae278afe41c11b41c949ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2185472Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67659}