i965/fs: Add support for removing MOV.NZ instructions.

i965/fs: Add support for removing MOV.NZ instructions.

For some reason, we occasionally write the flag register with a MOV.NZ
instruction:

   add(8)          g25<1>F         -g6<0,1,0>F     g15<8,8,1>F
   cmp.l.f0(8)     g26<1>D         g25<8,8,1>F     0F
   mov.nz.f0(8)    null            g26<8,8,1>D

A MOV.NZ instruction on the result of a CMP is like comparing for
equality with true in C. It's useless. Removing it allows us to
generate:

   add.l.f0(8)     null            -g6<0,1,0>F     g15<8,8,1>F

total instructions in shared programs: 5955701 -> 5951657 (-0.07%)
instructions in affected programs:     302910 -> 298866 (-1.34%)
GAINED:                                1
LOST:                                  0

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