Move I32x4SConvertF32x4 into shared implementation, and takes care of
both AVX and no-AVX implementation. Instruction selector still requires
dst == src to save a move in codegen.

Bug: v8:11589
Change-Id: Ie982682b3002192ab27700bf73f8c1e66aeba492
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3086732
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76243}