[frange] Add op2_range for operator_not_equal.

We're missing an op2_range entry for operator_not_equal so GORI can
calculate an outgoing edge.  The false side of != is true and
guarantees we don't have a NAN, so it's important to get this right.
We eventually get it through an intersection of various ranges in
ranger, but it's best to get things correct as early as possible.

gcc/ChangeLog:

	* range-op-float.cc (operator_not_equal::op2_range): New.
	* range-op-mixed.h: Add operator_not_equal::op2_range.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vrp-float-13.c: New test.