middle-end/93273 - fix sinking clobbers across backedges

middle-end/93273 - fix sinking clobbers across backedges

The previous work to fix PR93199 didn't take into account backedges
when defering insertion.  The following simply avoids to defer in that
case since we know we'll not take secondary opportunities there.

2020-01-15  Richard Biener  <rguenther@suse.de>

        PR middle-end/93273
        * tree-eh.c (sink_clobbers): If we already visited the destination
        block do not defer insertion.
        (pass_lower_eh_dispatch::execute): Maintain BB_VISITED for
        the purpose of defered insertion.

	* g++.dg/torture/pr93273.C: New testcase.