nir: Fix breakage of foreach_list_typed_safe assumptions in loop unrolling

foreach_list_typed_safe works with assumption that even if current node becomes invalid, the next will be still valid.

However process_loops broke this assumption, because during iteration when immediate child is unrolled - not only current node could be removed but also the one after it.

This doesn't cause issues now but it will cause issues when undefined behaviour in foreach* macros is fixed.

The fix is done in this way to avoid unrolling both outer and inner loop in one pass.

Last run on Intel CI (https://mesa-ci.01.org/global_logic/builds/250/group/63a9f0ea7bb98050796b649e85481845) had some issues probably not related to this commit, but I'll resend it again later.

Ci passes: https://mesa-ci.01.org/global_logic/builds/251/group/63a9f0ea7bb98050796b649e85481845

Also see !4129 (merged) for the fix in macros.