i965/fs: Emit MAD instructions when possible.

i965/fs: Emit MAD instructions when possible.

Previously we didn't emit MAD instructions since they cannot take
immediate arguments, but with the opt_combine_constants() pass we can
handle this properly.

total instructions in shared programs: 5920017 -> 5733278 (-3.15%)
instructions in affected programs:     3625153 -> 3438414 (-5.15%)
helped:                                22017
HURT:                                  870
GAINED:                                91
LOST:                                  49

Without constant pooling, this patch is a complete loss:

total instructions in shared programs: 5912589 -> 5987888 (1.27%)
instructions in affected programs:     3190050 -> 3265349 (2.36%)
helped:                                1564
HURT:                                  17827
GAINED:                                27
LOST:                                  101

And since the constant pooling patch by itself hurt a bunch of things,
from before constant pooling to this patch the results are:

total instructions in shared programs: 5895414 -> 5747946 (-2.50%)
instructions in affected programs:     3617993 -> 3470525 (-4.08%)
helped:                                20478
HURT:                                  4469
GAINED:                                54
LOST:                                  146

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