aco: fix spills_entry heuristic for branch blocks in init_live_in_vars()

aco: fix spills_entry heuristic for branch blocks in init_live_in_vars()

fossil-db (Navi):
Totals from 222 (0.16% of 135946) affected shaders:
SpillSGPRs: 9121 -> 9117 (-0.04%)
SpillVGPRs: 2820 -> 1821 (-35.43%)
CodeSize: 5134264 -> 5053336 (-1.58%); split: -1.63%, +0.05%
Instrs: 953435 -> 938761 (-1.54%); split: -1.59%, +0.05%
Cycles: 100567688 -> 97252432 (-3.30%); split: -3.34%, +0.04%
VMEM: 40752 -> 39219 (-3.76%); split: +0.04%, -3.80%
SMEM: 15416 -> 15509 (+0.60%); split: +0.64%, -0.03%
VClause: 20120 -> 19091 (-5.11%)
SClause: 23540 -> 23544 (+0.02%); split: -0.11%, +0.12%
Copies: 125912 -> 122017 (-3.09%); split: -3.36%, +0.26%
Branches: 31131 -> 30009 (-3.60%)

Mostly affects parallel-rdp ubershaders.

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>