i965/vs: Use the MAD instruction when possible.

i965/vs: Use the MAD instruction when possible.

This is different from how we do it in the FS - we are using MAD even when
some of the args are constants, because with the relatively unrestrained
ability to schedule a MOV to prepare a temporary with that data, we can
get lower latency for the sequence of instructions.

No significant performance difference on GLB2.7 trex (n=33/34), though it
doesn't have that many MADs.  I noticed MAD opportunities while reading
the code for the DOTA2 bug.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>