aco: reserve 2 sgprs for each branch

We'll need two sgprs for the possibility of a long jump.

fossil-db (Navi):
Totals from 10197 (7.50% of 135946) affected shaders:
SGPRs: 946268 -> 946468 (+0.02%)
VGPRs: 705884 -> 707956 (+0.29%); split: -0.00%, +0.30%
SpillSGPRs: 31485 -> 36212 (+15.01%); split: -0.04%, +15.05%
CodeSize: 88296484 -> 88384604 (+0.10%); split: -0.01%, +0.11%
MaxWaves: 81379 -> 81171 (-0.26%)
Instrs: 17219111 -> 17231682 (+0.07%); split: -0.03%, +0.10%
Cycles: 1594875900 -> 1596450136 (+0.10%); split: -0.05%, +0.15%
VMEM: 1687263 -> 1689080 (+0.11%); split: +0.14%, -0.03%
SMEM: 657726 -> 660262 (+0.39%); split: +0.61%, -0.22%
VClause: 294806 -> 294638 (-0.06%); split: -0.08%, +0.02%
SClause: 556702 -> 556210 (-0.09%); split: -0.12%, +0.03%
Copies: 1466323 -> 1469349 (+0.21%); split: -0.57%, +0.78%
Branches: 619793 -> 618556 (-0.20%); split: -0.28%, +0.08%
PreSGPRs: 806364 -> 811477 (+0.63%); split: -0.14%, +0.77%
PreVGPRs: 655845 -> 657174 (+0.20%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Cc: 20.2 <mesa-stable>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6212>