nir/loop_unroll: Do not unroll huge loops

Even force loop unroll should have some limits because it may want to unroll truly huge loop and make resulting native code exceed limits leading to crash.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111120
Fixes dEQP-VK.spirv_assembly.instruction.graphics.spirv_ids_abuse.lots_ids_*
Fixes: 51daccb2 ("nir: add a loop unrolling pass")