Allows the backend to rely on this bitset always being valid, instead of
working around the bug.

Bug: angleproject:5587
Change-Id: I25e1304c0e5e34b5fc1677a819315574603ed034
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2658885
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>