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 <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler