[Mesa-dev] [RFC] discard optimization and lowering passes

[Kenneth Graunke]

I was actually working on the same thing today, and wrote two passes for
dealing with discards.  I was just looking into adding ir_to_mesa and brw_fs
support when I saw your patch, so I just split that out and reused it.  Thanks
for writing that!

I'm not sure whether doing it in a separate pass (like I've done) or in
lower_if_to_cond_assign is better.

Also, I haven't been able to properly test any of this yet since I don't
yet have brw_fs support.  It may be completely broken, but I wanted to put it
out there anyway so people know there are two possible solutions.