Moves some checks into helper functions. Refactor only.

Bug: angleproject:4959
Change-Id: I08053a96f99baff75eb3954ecbb77e83483eba0f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2343406Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>