intel/fs: Don't try to copy propagate into a source again after progress is made

intel/fs: Don't try to copy propagate into a source again after progress is made

If the linked list structure used depended on the list head to know when
to terminate, this would be a pretty serious bug. If try_constant_propage
or try_copy_propagate make progress, inst->src[i].nr will change. This
results in the foreach_in_list using a different list header on later
iterations of the loop.

This causes two shaders in shader-db and 9 shaders in fossil-db to
change. Looking at the code changes, these are cases where there was a
copy of a copy that gets propagated. The part that confuses me is the
VGRF numbers involved should **not** hash to the same bucket, so it
should be impossible to find the original source from the intermediate
VGRF.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25091>