When iterating over a bitset of stages, they are now processed in the
same order as the rendering pipeline.

Bug: angleproject:5405
Change-Id: If9e0f24be047b060f759e0e1db622a91e24e9646
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2564008Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>