Warn on undefined loop exit.

2014-11-20  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* tree-ssa-loop-niter.c (maybe_lower_iteration_bound): Warn if a loop
	condition would be removed due to undefined behaviour.

	gcc/testsuite/
	* gcc.dg/undefined-loop-1.c: New file.
	* gcc.dg/undefined-loop-2.c: New file.

From-SVN: r217891