nir: Move vc4's alpha test lowering to core NIR.

nir: Move vc4's alpha test lowering to core NIR.

I've been doing this inside of vc4, but vc5 wants it as well and it may be
useful for other drivers (Intel has a related path for pre-gen6 with MRT,
and freedreno had a TGSI path for it at one point).

This required defining a common enum for the standard comparison
functions, but other lowering passes are likely to also want that enum.

v2: Add to meson.build as well.

Acked-by: Rob Clark <robdclark@gmail.com>