i965/fs: Add a pass to fixup 3-src instructions that have a null dest.

i965/fs: Add a pass to fixup 3-src instructions that have a null dest.

3-src instructions can only have GRF/MRF destinations. It's really
difficult to deal with that restriction in dead code elimination (that
wants to give instructions null destinations to show that their result
isn't used) while allowing 3-src instructions to have conditional mod,
so don't, and just give then a destination before register allocation.

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