[range-op-float] Abstract out binary operator code out of PLUS_EXPR entry.

The PLUS_EXPR was always meant to be a template for further
development, since most of the binary operators will share a similar
structure.  This patch abstracts out the common bits into the default
definition for range_operator_float::fold_range() and provides an
rv_fold() to be implemented by the individual entries wishing to use
the generic folder.  This is akin to what we do with fold_range() and
wi_fold() in the integer version of range-ops.

gcc/ChangeLog:

	* range-op-float.cc (range_operator_float::fold_range): Abstract
	out from foperator_plus.
	(range_operator_float::rv_fold): New.
	(foperator_plus::fold_range): Remove.
	(foperator_plus::rv_fold): New.
	(propagate_nans): Remove.
	* range-op.h (class range_operator_float): Add rv_fold.