tree-optimization/116879 - failure to recognize non-empty latch

tree-optimization/116879 - failure to recognize non-empty latch

When we relaxed the vectorizers constraint on loop structure verifying
the emptiness of the latch became too lose as can be seen in the case
for PR116879 where the latch effectively contains two basic-blocks
which one being an unmerged forwarder that's not empty.

	PR tree-optimization/116879
	* tree-vect-loop.cc (vect_analyze_loop_form): Scan all
	blocks that form the latch.

	* gcc.dg/pr116879.c: New testcase.