The {InstructionCode} is only used to store plain (non-negative) values
of the {ArchOpcode} enum, or additionally encodes {BitField} values. The
underlying base type 'U' of a {BitField} is uint32_t. To avoid all the
numerous implicit conversions between int32_t and uint32_t, this is
changing the {InstructionCode} so that uint32_t is used exclusively.

R=neis@chromium.org
BUG=v8:9872

Change-Id: If64107ad9298011e219b4827735eafb51465beb0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1869193Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64503}