Only keep Memory-Phi nodes.

This fixes opt/fehler261.c.
jumpthreading: cleanup
jumpthreading SSA reconstruction must add keepalive edges to PhiM nodes
iropt: cleanup transform_node_Phi
assert that we are not exchanging kept PhiM nodes
remove keep edge when eliminating PhiM in transform_node_Phi()
transfer keep-alive edge if necessary in transform_node_Phi
ldstop: transfer keep-alive edge if necessary
fix equivalent_node_Phi()

A self-loop before the first real value was not recognized before and
could lead to an invalid kept-PhiM removal.
cfopt: be less aggressive when removing moved phis

This should fix the case where it would remove an unused PhiM in an endless
loop (which is not correct).