We can potentially work around a bug in the HLSL compiler by
omitting the const qualifier from some sampling operations.

BUG=angleproject:1923

Change-Id: I8a5d119707721e9c19f06be4ad808f87bfcdbee5
Reviewed-on: https://chromium-review.googlesource.com/454938Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>