nouveau/nv50: disable GLSL IR loop unrolling

nouveau/nv50: disable GLSL IR loop unrolling

NIR loop unrolling is already enabled so just let it do its job.

Shader-db results (nv92):

total gpr in shared programs: 734638 -> 735037 (0.05%)
gpr in affected programs: 11058 -> 11457 (3.61%)
total instructions in shared programs: 6073415 -> 6073398 (<.01%)
instructions in affected programs: 10079 -> 10062 (-0.17%)
total bytes in shared programs: 41837432 -> 41838872 (<.01%)
bytes in affected programs: 252504 -> 253944 (0.57%)

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16366>