pretty-ipa merge 17: CD-DCE wrt cyclic CFGs fix

[Jan Hubicka]

Hi,
did we get some conclusion about this patch?  There are real bugs in
empty loop removal that can be seen when compiling testsuite with
-funsafe-loop-transformations (it ICEs when removing loop exited via
SWITCH statement for example).
So I would like to remove it in favour of teaching CD-DCE aboutloops.
It adds need to fire scev up one extra time (during early optimizations)
but I do need empty loop removal in early pass anyway (it happens in
pooma code commonly, not only tramp3d after inlining simple
constructors).
We can get better definition of control dependency later, but that is
followup anyway :)

Honza