nir: Optimize out discard_ifs with a constant 0 argument.
nir: Optimize out discard_ifs with a constant 0 argument.

I found this in a shader that was doing an alpha test when alpha is fixed
at 1.0.

v2: Rebase on master (now the const value is "u32" not "u").

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net> (v1)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler