Added the isSamplerRegister() function to bring back the previous
validation whose removal was causing uniform-default-values.html
to fail, but I added a comment to explain why this validation was
necessary in the first place.

BUG=18029434

Change-Id: I9dbbfdd31c45adffd5e14433b2ace3a1f79a1ff0
Reviewed-on: https://swiftshader-review.googlesource.com/1471Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>