Adds a new bitset helper and changes a check to an ASSERT.
Refactoring change only.

Bug: angleproject:4959
Change-Id: I0de9f1b707c87cfb6fed8a110654783059e55c99
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2345025
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>