On a6xx/a5xx there is such dependency between branchstack bitfield
and the amount of nested ifs, which could be seen with blob:

IFs   BRANCHSTACK
0	0
1	1
2	2
3	2
4	3
5	3
6	4
...
59	30
60	31
61	31
62	32
63	32
64	32

Remove open-coded branchstack for a5xx compute along the way.

Fixes tests:
 dEQP-VK.spirv_assembly.instruction.compute.float16.opvectorshuffle.344
 dEQP-VK.spirv_assembly.instruction.graphics.float16.opvectorshuffle.344_vert
 dEQP-VK.spirv_assembly.instruction.graphics.float16.opvectorshuffle.444_geom
 dEQP-VK.spirv_assembly.instruction.graphics.float16.opvectorshuffle.244_tessc
 dEQP-VK.spirv_assembly.instruction.graphics.float16.opvectorshuffle.344_frag

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <mesa/mesa!9859>