tree-ssa.c (useless_type_conversion_p): Document future intent as defining the middle-end type system.

2007-07-02  Richard Guenther  <rguenther@suse.de>

	* tree-ssa.c (useless_type_conversion_p): Document
	future intent as defining the middle-end type system.
	Re-structure to call langhook last, group by type class,
	mark questionable parts.

From-SVN: r126199