The overly-restrictive check would fail on sampler types or other
GL types with no defined precision.

This fixes failures on the Debug GPU FYI bots.

BUG=angle:691

Change-Id: I443b7038a7b7a05675898b9b21e6112b1ac4c5ca
Reviewed-on: https://chromium-review.googlesource.com/206552Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>