The concern originally was that rewriting sampler structs could lead to
incorrect behaviour for indexing. The test attempts to cover this but
doesn't repro. It possibly is not an issue.

Bug: angleproject:2494
Change-Id: Ibc34b08b5cee3b6ff82d150a64f1768aae64396f
Reviewed-on: https://chromium-review.googlesource.com/1117321Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>