tree-optimization/45178 - DCE of dead control flow in infinite loop

tree-optimization/45178 - DCE of dead control flow in infinite loop

This fixes DCE to be able to elide dead control flow in an
infinite loop without an exit edge.  This special situation is
handled well by the code finding an edge to preserve since there's
no chance it will find the exit edge and make the loop finite.

2021-08-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/45178
	* tree-ssa-dce.c (find_obviously_necessary_stmts): For
	infinite loops without exit do not mark control dependent
	edges of the latch necessary.

	* gcc.dg/tree-ssa/ssa-dce-3.c: Adjust testcase.