i965/fs: Cut an instruction and a temporary from gen6 discard statements.

i965/fs: Cut an instruction and a temporary from gen6 discard statements.

I thought I was thwarted initially when I couldn't do conditional mod
on a MOV, and couldn't use two immediate constants in one instruction.
But g0 != g0 is also a way to produce a failing comparison.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>