Handling RGB10A2 and A2BGR10 formats are made through dynamic flags.
One or two `if` checks (based on format) is not worth doubling the
number of shaders.

Shaves about 50KB from binary size.

Bug: angleproject:2022
Bug: chromium:1084580
Change-Id: Ia603a935b66d002dbacdee9342950ceed4042f86
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2240672Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>