i965/fs: Add a saturation propagation optimization pass.

i965/fs: Add a saturation propagation optimization pass.

Transforms, for example,

mul     vgrf3, vgrf2, vgrf1
mov.sat vgrf4, vgrf3

into

mul.sat vgrf3, vgrf2, vgrf1
mov     vgrf4, vgrf3

which gives register_coalescing an opportunity to remove the MOV
instruction.

total instructions in shared programs: 1515039 -> 1504634 (-0.69%)
instructions in affected programs:     798586 -> 788181 (-1.30%)
GAINED:                                0
LOST:                                  4

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>