Don't use byte-width instructions when accessing |compiler_hints| field.
This CL eases adding new bit fields to the compiler hints field.

Bug: v8:6470
Change-Id: I7b07c1c8d0a11a303eebb5272d2846a5a84005f7
Reviewed-on: https://chromium-review.googlesource.com/529804
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45860}