Fix multisample texture operations crashing HLSL generation This includes a partial implementation of multisample texture operations on the HLSL backend. It can't be fully tested yet, since the API side isn't implemented. BUG=angleproject:1442 TEST=dEQP-GLES31.functional.shaders.builtin_functions.texture_size.* (successfully compiles instead of crashing) Change-Id: Ief782db28388a3f8fd8113cc86ce3c4f500f322a Reviewed-on: https://chromium-review.googlesource.com/443264 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>