tree-optimization/114787 - more careful loop update with CFG cleanup

When CFG cleanup removes a backedge we have to be more careful with
loop update.  In particular we need to clear niter info and estimates
and if we remove the last backedge of a loop we have to also mark
it for removal to prevent a following basic block merging to associate
loop info with an unrelated header.

	PR tree-optimization/114787
	* tree-cfg.cc (remove_edge_and_dominated_blocks): When
	removing a loop backedge clear niter info and when removing
	the last backedge of a loop mark that loop for removal.

	* gcc.dg/torture/pr114787.c: New testcase.