This fixes cases where a matrix is indexed and then the resulting
vector is indexed in the same expression inside an l-value.

BUG=angleproject:1672
TEST=angle_end2end_tests

Change-Id: Ia0593d75da67e0aa582003e1c381f2a1c89f948c
Reviewed-on: https://chromium-review.googlesource.com/427938Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>