broadcom/compiler: improve allocation for final program instructions

broadcom/compiler: improve allocation for final program instructions

The last 3 instructions can't use specific registers so flag all the
nodes for temps used in the last program instructions and try to
avoid assigning any of these. This may help us avoid injecting nops
for the last thread switch instruction.

Because regisster allocation needs to happen before QPU scheduling
and instruction merging we can't tell exactly what the last 3
instructions will be, so we do this for a few more instructions than
just 3.

We only do this for fragment shaders because other shader stages
always end with VPM store instructions that take an small immediate
and therefore will never allow us to merge the final thread switch
earlier, so limiting allocation for these shaders will never improve
anything and might instead be detrimental.

total instructions in shared programs: 11471389 -> 11464335 (-0.06%)
instructions in affected programs: 582908 -> 575854 (-1.21%)
helped: 4669
HURT: 578
Instructions are helped.

total max-temps in shared programs: 2230497 -> 2230150 (-0.02%)
max-temps in affected programs: 5662 -> 5315 (-6.13%)
helped: 344
HURT: 44
Max-temps are helped.

total sfu-stalls in shared programs: 18068 -> 18077 (0.05%)
sfu-stalls in affected programs: 264 -> 273 (3.41%)
helped: 37
HURT: 48
Inconclusive result (value mean confidence interval includes 0).

total inst-and-stalls in shared programs: 11489457 -> 11482412 (-0.06%)
inst-and-stalls in affected programs: 585180 -> 578135 (-1.20%)
helped: 4659
HURT: 588
Inst-and-stalls are helped.

total nops in shared programs: 301738 -> 298140 (-1.19%)
nops in affected programs: 14680 -> 11082 (-24.51%)
helped: 3252
HURT: 108
Nops are helped.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25450>