aco/jump_threading: don't remove loop preheaders

They might be needed as convergence point in order to
insert code (e.g. for loop alignment, wait states, etc.).

Totals from 1 (0.00% of 79395) affected shaders:

CodeSize: 12672 -> 12716 (+0.35%)
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32477>
1 file changed