Introduce rtx_sequence subclass of rtx_def

Introduce rtx_sequence subclass of rtx_def

gcc/
2014-08-27  David Malcolm  <dmalcolm@redhat.com>

	* coretypes.h (class rtx_sequence): Add forward declaration.
	* rtl.h (class rtx_sequence): New subclass of rtx_def, adding
	invariant: GET_CODE (X) == SEQUENCE.
	(is_a_helper <rtx_sequence *>::test): New.
	(is_a_helper <const rtx_sequence *>::test): New.
	(rtx_sequence::len): New.
	(rtx_sequence::element): New.
	(rtx_sequence::insn): New.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@214591 138bc75d-0d04-0410-961f-82ee72b054a4