This is a prepratory refactor for converting index buffers on the GPU
using a more generic compute shader. No functional change.

Bug: angleproject:3490
Change-Id: Iadf4b1429314db6850320aee33c4113f38577378
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1639057Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>