We don't need to explicitly check if a barrier is required for write
barriers. Write barriers always require a barrier and read barriers
need the layout change check. We introduce a new enum encoding ReadOnly
vs Write layout types and call specialized write/read functions
instead. Also renames the helper APIs to be more consistent.

Refactoring change only.

Bug: angleproject:4959
Change-Id: I0ce39ceaca6be588327c381194a580dc6b11f036
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2344744
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>