In prepration for EXT_copy_image taking advantage of ConvertVertex.
Also a small optimization to this ALU heavy shader.

Bug: angleproject:5278
Change-Id: I6520391fe1af608db66f8c362f1038786b2c65e7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2556466Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>