nir: Add unit tests for nir_opt_comparison_pre

Each tests has a comment with the expected before and after NIR.  The
tests don't actually check this.  The tests only check whether or not
the optimization pass reported progress.  I couldn't think of a robust,
future-proof way to check the before and after code.

Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit b08d7040)