[range-op-float] Implement MINUS_EXPR.

Now that the generic parts of the binary operators have been
abstracted, implementing MINUS_EXPR is a cinch.

The op[12]_range entries will be submitted as a follow-up.

gcc/ChangeLog:

	* range-op-float.cc (class foperator_minus): New.
	(floating_op_table::floating_op_table): Add MINUS_EXPR entry.