[MemProf] Remove empty edges once after cloning (#85320)

Restructure the handling of edges that become empty during the cloning
process. Instead of removing them as they become empty (no context ids
and alloc type), do this once after all cloning is complete.

This has no effect on the cloning result, but prepares for a follow on
change that does improve the cloning. The structural change here reduces
the diffs for the follow on change, which would be much more difficult
with the previous handling.