vc4: Allow merging instructions with SF set where the other writes NOP.

vc4: Allow merging instructions with SF set where the other writes NOP.

I'm not sure how I managed to write the SF merge code
(7d8b79f398f18ed7bb48a74b1b82950e2f08abad) without allowing merges with
NOPs.  *Everything* we try to merge with will have a NOP on one or the
other side of the instruction, and that's why that commit showed no
benefit.

total instructions in shared programs: 99347 -> 95128 (-4.25%)
instructions in affected programs:     91906 -> 87687 (-4.59%)

3DMMES performance +2.57105% +/- 0.135276% (n=6,8)