broadcom/compiler: don't always assign r5 if available

broadcom/compiler: don't always assign r5 if available

Instead, only favor assigning r5 if we have first decided to
assign an accumulator. This helps with assining r5 to short
lived uniforms, favoring accumulator rotation to facilitate
QPU merges.

total instructions in shared programs: 12656164 -> 12628339 (-0.22%)
instructions in affected programs: 5368373 -> 5340548 (-0.52%)
helped: 17420
HURT: 9996

total uniforms in shared programs: 3704776 -> 3704863 (<.01%)
uniforms in affected programs: 12247 -> 12334 (0.71%)
helped: 23
HURT: 78

total max-temps in shared programs: 2153505 -> 2152684 (-0.04%)
max-temps in affected programs: 26468 -> 25647 (-3.10%)
helped: 569
HURT: 328

total fills in shared programs: 4656 -> 4657 (0.02%)
fills in affected programs: 43 -> 44 (2.33%)
helped: 0
HURT: 1

total sfu-stalls in shared programs: 34728 -> 34403 (-0.94%)
sfu-stalls in affected programs: 3411 -> 3086 (-9.53%)
helped: 842
HURT: 534

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15276>