Add support for access to gl_SampleMask or gl_SampleMaskIn
with non-constant index. Replace gl_SampleMask with
ANGLESampleMask and reassign ANGLESampleMask
to gl_SampleMask in the end.

Bug: angleproject:3588
Tests: dEQP-GLES31.functional.shaders.sample_variables.sample_mask.
           discard_half_per_pixel.*
           discard_half_per_sample.*
Change-Id: Ie668c3ccf4d40352574db57f5e43b2f17ef6d8b8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2477908
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>