Use attributes to avoid unrolling and flatten conditionals.

[loop] and [flatten] help avoid unrolling make more shaders compile
successfully. When unrolling has to happen, an error X3531 is generated
so we have to try compiling again without these attributes.

BUG=395048,395286

Change-Id: I91ea8ac3a822ab1f1ac24907d42326e3c6c9d9af
Reviewed-on: https://chromium-review.googlesource.com/222810
Tested-by: Nicolas Capens <capn@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>