Introduce rtx_insn subclass of rtx_def

Introduce rtx_insn subclass of rtx_def

gcc/
2014-08-18  David Malcolm  <dmalcolm@redhat.com>

	* coretypes.h (class rtx_insn): Add forward declaration.

	* rtl.h: Include is-a.h.
	(struct rtx_def): Add dummy "desc" and "tag" GTY options as a
	workaround to ensure gengtype knows inheritance is occurring,
	whilst continuing to use the pre-existing special-casing for
	rtx_def.
	(class rtx_insn): New subclass of rtx_def, adding the
	invariant that we're dealing with something we can sanely use
	INSN_UID, NEXT_INSN, PREV_INSN on.
	(is_a_helper <rtx_insn *>::test): New.
	(is_a_helper <const rtx_insn *>::test): New.

From-SVN: r214118