[Bug middle-end/38474] compile time explosion in dataflow_set_preserve_mem_locs at -O3

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38474

--- Comment #91 from Richard Biener <rguenth at gcc dot gnu.org> ---
So the other simple idea I have is to limit the number of active store groups
and force-terminate in either a LRU or FIFO manner.

For the testcase at hand the decls we start the chain for are all only
used in full but knowing that would require some pre-analysis of the IL
similar to what SRA does for example (collecting all accesses).  It's
then also still easy to "break" such heuristic so limiting is in the end
the only (and I guess best) option.