This will allow us to use the same code path for assigning register
information in all places. Because of this it fixes some long-broken
dEQP struct uniform tests.

BUG=angle:466
BUG=angle:505

Change-Id: I4161a388503aa09bbe2d21ff47bfb3352ec93881
Reviewed-on: https://chromium-review.googlesource.com/207255Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>