i965/fs: Readd opt_drop_redundant_mov_to_flags().

i965/fs: Readd opt_drop_redundant_mov_to_flags().

This reverts commit b449366587b5f3f64c6fb45fe22c39e4bc8a4309.

I removed the pass thinking that it was now not useful, but that was not
true. I believe I ran shader-db on HSW and saw no results, but HSW does
not use the unlit centroid workaround code and as a result does not emit
redundant MOV_DISPATCH_TO_FLAGS instructions.

On IVB, the shader-db results are:

total instructions in shared programs: 6650806 -> 6646303 (-0.07%)
instructions in affected programs: 106893 -> 102390 (-4.21%)
helped: 793

total cycles in shared programs: 56195538 -> 56103720 (-0.16%)
cycles in affected programs: 873048 -> 781230 (-10.52%)
helped: 553
HURT: 209

On SNB, the shader-db results are:

total instructions in shared programs: 7173074 -> 7168541 (-0.06%)
instructions in affected programs: 119757 -> 115224 (-3.79%)
helped: 799

total cycles in shared programs: 98128032 -> 98072938 (-0.06%)
cycles in affected programs: 1437104 -> 1382010 (-3.83%)
helped: 454
HURT: 237

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>