ptn: Emit mul+add for MAD

ptn: Emit mul+add for MAD

Unlike fma() in GLSL, MAD in ARB programs is 100% splittable.  Just emit
the split version and let the optimizer fuse them later.

Shader-db results on Haswell:

   total instructions in shared programs: 7560379 -> 7560300 (-0.00%)
   instructions in affected programs: 143928 -> 143849 (-0.05%)
   helped: 443
   HURT: 250

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