nir: Rename opt_undef_alu to opt_undef_csel; update comments.

nir: Rename opt_undef_alu to opt_undef_csel; update comments.

This better reflects what it does.  I plan to add other ALU
optimizations as well, so the old name would be confusing.

In preparation for that, also move the file comments about csels
above the opt_undef_csel function, and delete the ones about there
not being other optimizations.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>