nir: handle negatives in ffma reassociation optimisation

Noticed this when looking into some regressions from other changes I'm working on.

shader-db results Iris (BDW):

total instructions in shared programs: 16632076 -> 16631057 (<.01%)
instructions in affected programs: 48010 -> 46991 (-2.12%)
helped: 47
HURT: 6

total cycles in shared programs: 915266726 -> 915263622 (<.01%)
cycles in affected programs: 1182283 -> 1179179 (-0.26%)
helped: 18
HURT: 27

total loops in shared programs: 4929 -> 4929 (0.00%)
loops in affected programs: 0 -> 0
helped: 0
HURT: 0

total spills in shared programs: 18834 -> 18801 (-0.18%)
spills in affected programs: 525 -> 492 (-6.29%)
helped: 3
HURT: 0

total fills in shared programs: 23008 -> 22981 (-0.12%)
fills in affected programs: 435 -> 408 (-6.21%)
helped: 3
HURT: 0