nir/legacy: Fix fneg(load_reg) case

nir/legacy: Fix fneg(load_reg) case

Consider the IR:

   %0 = load_reg
   %1 = fneg %0
   %2 = ffloor %1
   %3 = bcsel .., .., %1

Because the fneg has both foldable and non-foldable users, nir/legacy does not
fold the fneg into the load_reg. This ensures that the backend correctly emits a
dedicated fneg instruction (with the load_reg folded in) for the bcsel to use.
However, because the chasing helpers did not previously take other uses of a
modifier into account, the helpers would fuse in the fneg to the ffloor. Except
that doesn't work, because the load_reg instruction is supposed to be
eliminated. So we end up with broken chased IR:

   1 = fneg r0
   2 = ffloor -NULL
   3 = bcsel, ..., 1

The fix is easy: only fold modifiers into ALU instructions if the modifiers can
be folded away. If we can't eliminate the modifier instruction altogether, it's
not necessarily beneficial to fold it anyway from a register pressure
perspective. So this is probably ok. With that check in place we get correct IR

   1 = fneg r0
   2 = ffloor 1
   3 = bcsel, ..., 1

Fixes carchase/230.shader_test under softpipe.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24116>