* omp-low.c (extract_omp_for_data): For collapsed loops,
	if at least one of the loops is known at compile time to
	iterate zero times, set count to 0.
	(expand_omp_regimplify_p): New function.
	(expand_omp_for_generic): For collapsed loops, if at least
	one of the loops isn't known to iterate at least once,
	add runtime check with setting count to 0.
	(expand_omp_for_static_nochunk, expand_omp_for_static_chunk):
	For unsigned types if it isn't known at compile time that
	the loop will iterate at least once, add runtime check to bypass
	the whole loop if initial condition isn't true.

	* testsuite/libgomp.c/loop-13.c: New test.
	* testsuite/libgomp.c/loop-14.c: New test.
	* testsuite/libgomp.c/loop-15.c: New test.
	* testsuite/libgomp.c++/loop-13.C: New test.
	* testsuite/libgomp.c++/loop-14.C: New test.
	* testsuite/libgomp.c++/loop-15.C: New test.

From-SVN: r198966