* tree-ssa-loop-niter.c (implies_nonnegative_p): New function.
	(derive_constant_upper_bound): Derive more precise upper bound in
	common cases.  Return type changed to double_int.
	(record_estimate): Reflect the changed return type of
	derive_constant_upper_bound.
	* double-int.c (double_int_zext, double_int_sext): Fix.

	* gcc.dg/tree-ssa/loop-18.c: New test.

From-SVN: r114674