Cleanups to frange.

These are some assorted cleanups to the frange class to make it easier
to drop in an implementation with FP endpoints:

* frange::set() had some asserts limiting the type of arguments
  passed.  There's no reason why we can't handle all the variants.
  Worse comes to worse, we can always return a VARYING which is
  conservative and correct.

* frange::normalize_kind() now returns a boolean that can be used in
  union and intersection to indicate that the range changed.

* Implement vrp_val_max and vrp_val_min for floats.  Also, move them
  earlier in the header file so frange can use them.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* value-range.cc (tree_compare): New.
	(frange::set): Make more general.
	(frange::normalize_kind): Cleanup and return bool.
	(frange::union_): Use normalize_kind return value.
	(frange::intersect): Same.
	(frange::verify_range): Remove unnecessary else.
	* value-range.h (vrp_val_max): Move before frange class.
	(vrp_val_min): Same.
	(frange::frange): Remove set to m_type.