This moves several loop checks outside the loop, and uses more of the
gl::AttributesMask class to do bitset operations instead of using
checks and for loops.

Bug: angleproject:1391
Change-Id: I90a1a7db550390ecd1402cf5a8a6677fd852b7b0
Reviewed-on: https://chromium-review.googlesource.com/1008273Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>