tree-optimization/118973 - stray abnormal edge after DCE

DCE preserves stmts performing abnormal control flow transfer but
currently has an exception for replaceable allocations and cxa_atexit
calls.  That results in a broken CFG since DCE isn't set up to prune
abnormal edges possibly hanging off those.

While we could try to add this handling, the following is the safe
fix at this point and more suitable for backporting.

	PR tree-optimization/118973
	* tree-ssa-dce.cc (mark_stmt_if_obviously_necessary): Calls
	that alter control flow in unpredictable ways need to be
	preserved.

	* g++.dg/torture/pr118973.C: New testcase.