[Bug target/108738] compile-time and memory-hog in mdreorg

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108738

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
The odd thing is that DF_REF_CHAIN of a USE ref contains _all_ definitions of
the pseudo while DF_REF_CHAIN of a DEF ref contains only reaching uses!?

Probably an artifact of how df_chain_create_bb_process_use operates.

That makes this much harder to fix ...