pan/bi: Treat +DISCARD.f32 as message-passing

pan/bi: Treat +DISCARD.f32 as message-passing

Likely errata, matches blob's handling. Closes #4387

total nops in shared programs: 86266 -> 86272 (<.01%)
nops in affected programs: 347 -> 353 (1.73%)
helped: 1
HURT: 2

total clauses in shared programs: 20813 -> 20833 (0.10%)
clauses in affected programs: 343 -> 363 (5.83%)
helped: 0
HURT: 20
Clauses are HURT.

total quadwords in shared programs: 91572 -> 91588 (0.02%)
quadwords in affected programs: 1322 -> 1338 (1.21%)
helped: 1
HURT: 14
Quadwords are HURT.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Tested-by: Icecream95 <ixn@disroot.org>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9446>