Define compiler hints bits as bit fields but not as enum.
This eases adding new bit fields to the compiler hints field.

TBR=marja@chromium.org

Bug: v8:6470
Change-Id: Ia023bab25b925847924c45d5a57f85a14c8f1971
Reviewed-on: https://chromium-review.googlesource.com/529044
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45825}