Most were using uint8_t already, but some were declared as int8_t. This
CL consistently makes 8-bit immediates unsigned values.

R=titzer@chromium.org

Change-Id: I8f829486fb5ab1bf597b0be1eabc9b811543c3e3
Reviewed-on: https://chromium-review.googlesource.com/1220147Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55820}