The fuzzer uncovered a case where we were trying to emulate precision
on HLSL 3.0, causing an ASSERT crash.

BUG=chromium:653276

Change-Id: I2e666a1ff4f605541e25f04264146063559cb835
Reviewed-on: https://chromium-review.googlesource.com/394237Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>