Implement op[12]_range operators for PLUS_EXPR and MINUS_EXPR.

We can implement the op[12]_range entries for plus and minus in terms
of each other.  These are adapted from the integer versions.

gcc/ChangeLog:

	* range-op-float.cc (foperator_plus::op1_range): New.
	(foperator_plus::op2_range): New.
	(foperator_minus::op1_range): New.
	(foperator_minus::op2_range): New.