store-merging: Fix up a -fcompare-debug bug in get_status_for_store_merging [PR104263]

store-merging: Fix up a -fcompare-debug bug in get_status_for_store_merging [PR104263]

As mentioned in the PRthe following testcase fails, because the last
stmt of a bb with -g is a debug stmt and get_status_for_store_merging
uses gimple_seq_last_stmt (bb_seq (bb)) when testing if it is valid
for store merging.  The debug stmt isn't valid, while a stmt at that
position with -g0 is valid and so the divergence.

As we walk the whole bb already, this patch just remembers the last
non-debug stmt, so that we don't need to skip backwards debug stmts at the
end of the bb to find last real stmt.

2022-01-28  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/104263
	* gimple-ssa-store-merging.cc (get_status_for_store_merging): For
	cfun->can_throw_non_call_exceptions && cfun->eh test whether
	last non-debug stmt in the bb is store_valid_for_store_merging_p
	rather than last stmt.

	* gcc.dg/pr104263.c: New test.