broadcom/compiler: don't allocate undef to rf0

broadcom/compiler: don't allocate undef to rf0

rf0 is affected by restrictions in some scenarios so we rather use
a register that does not cause conflicts for scheduling.

total instructions in shared programs: 12850958 -> 12848024 (-0.02%)
instructions in affected programs: 331974 -> 329040 (-0.88%)
helped: 2559
HURT: 201
Instructions are helped.

total max-temps in shared programs: 2210893 -> 2210803 (<.01%)
max-temps in affected programs: 1486 -> 1396 (-6.06%)
helped: 96
HURT: 7
Max-temps are helped.

total sfu-stalls in shared programs: 21975 -> 21965 (-0.05%)
sfu-stalls in affected programs: 32 -> 22 (-31.25%)
helped: 16
HURT: 6
Sfu-stalls are helped.

total inst-and-stalls in shared programs: 12872933 -> 12869989 (-0.02%)
inst-and-stalls in affected programs: 332036 -> 329092 (-0.89%)
helped: 2560
HURT: 189
Inst-and-stalls are helped.

total nops in shared programs: 305911 -> 303501 (-0.79%)
nops in affected programs: 11215 -> 8805 (-21.49%)
helped: 2131
HURT: 3
Nops are helped.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22797>