glsl: replace unreachable code with an assert()

glsl: replace unreachable code with an assert()

All interface blocks will have been lowered by this point so just
use an assert. Returning false would have caused all sorts of
problems if they were not lowered yet and there is an assert to
catch this later anyway.

We also update the tests to reflect this change.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>