This tnodifies CodeStubAssembler::Float64ToUint8Clamped and
Int32ToUint8Clamped.

Bug: v8:6949
Change-Id: I79c8cd45dc4c8333f268cf0a62234d75369f89b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1813024
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63922}