nir/builder: Eliminate nir_f2b helper (and use of nir_f2b32 helper)

nir/builder: Eliminate nir_f2b helper (and use of nir_f2b32 helper)

There were only two users. Replace each with nir_fneu instead.

This is now a squash of what was two separate commits.
nir_lower_pstipple_block is called after nir_lower_bool_to_int32, so
nir_fneu32 has to be used here or there will be regresssions in stipple
tests on llvmpipe.

v2: Rebase on !20869.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Suggested-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20509>