Narrowing operations need to sign extend the result.

E.g. for narrowing uint16 to uint8, we compare uint16 to uint8 max,
0xff. The final result should be 0xffffffff (sign extended) since we
try to keep nodes in their sign extended form, to work well with
the rest of the lowering operations.

With this, we pass the last spec test (that is not ignored),
simd_conversions.

Bug: v8:10507
Change-Id: I8914fd69db9378b8244cba5dcacff98d36893649
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2436613Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70272}