i965/fs: Add support for generating MADs.

i965/fs: Add support for generating MADs.

Improves nexuiz performance 0.65% +/- .10% (n=5) on my gen6, and .39%
+/- .11% (n=10) on gen7.  No statistically significant performance
difference on warsow (n=5, but only one shader has MADs).

v2: Add support for MADs in 16-wide by using compression control.
v3: Don't generate MADs when it will force an immediate to be moved to a temp.
    (it's not clear whether this is a win or not, but it should result in less
     questionable change to codegen compared to v2).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v2)