tree-optimization/114052 - consider infinite sub-loops when lowering iter bound

When we walk stmts to find always executed stmts with UB in the last
iteration to be able to reduce the iteration count by one we fail
to consider infinite subloops in the last iteration that would make
such stmt not execute.  The following adds this.

	PR tree-optimization/114052
	* tree-ssa-loop-niter.cc (maybe_lower_iteration_bound): Check
	for infinite subloops we might not exit.

	* gcc.dg/pr114052-1.c: New testcase.