freedreno/ir3: eliminate unnecessary absneg's

freedreno/ir3: eliminate unnecessary absneg's

The frontend inserts (abs) and (neg)'s to convert between NIR boolean
(~0/0) and native boolean (1/0).  So we'd end up with things like:

   cmps.s.ge r1.x, ...
   absneg.s r1.x, (neg)r1.x
   absneg.s r1.x, (abs)r1.x
   sel.b32 r2.x, r0.x, r1.x, r0.y

The (neg) already gets collapsed due to the following (abs).  Now by
realizing that r1.x comes from a cmps.s instruction, we can drop the
(abs) as well.

Signed-off-by: Rob Clark <robclark@freedesktop.org>