We were hitting an implementation defined behavior in this instruction:

- v is clamped to uint8_t::min and uint8_t::max
- then we static_cast<int8_t>(v)
- any values that don't fit in int8_t (> 127) hits and implementation
defined behavior

We reuse base::saturated_cast here instead to avoid this undefined
behavior.

Drive-by cleanup of test cases to make the signed/unsigned cases more
explicity.

Bug: v8:11372
Change-Id: I4e92cdfb685d74bd5436ba25f1c00db49a231221
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2659501
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72491}