aco/ra: fix register allocation of branch definitions

aco/ra: fix register allocation of branch definitions

fossil-db (Sienna Cichlid):
Totals from 704 (0.52% of 134913) affected shaders:
CodeSize: 7177288 -> 7182072 (+0.07%); split: -0.00%, +0.07%
Instrs: 1371781 -> 1372977 (+0.09%); split: -0.00%, +0.09%
Latency: 17993572 -> 18001344 (+0.04%); split: -0.00%, +0.04%
InvThroughput: 4198996 -> 4199569 (+0.01%); split: -0.00%, +0.01%
Copies: 122456 -> 123516 (+0.87%); split: -0.01%, +0.88%
Branches: 43815 -> 43818 (+0.01%); split: -0.02%, +0.03%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13432>