Support constants and externals in BB reduction vectorization

Support constants and externals in BB reduction vectorization

The following supports vectorizing BB reductions involving a
constant or an invariant.

	* tree-vectorizer.h (_slp_instance::remain_stmts): Change
	to ...
	(_slp_instance::remain_defs): ... this.
	(SLP_INSTANCE_REMAIN_STMTS): Rename to ...
	(SLP_INSTANCE_REMAIN_DEFS): ... this.
	(slp_root::remain): New.
	(slp_root::slp_root): Adjust.
	* tree-vect-slp.cc (vect_free_slp_instance): Adjust.
	(vect_build_slp_instance): Get extra remain parameter,
	adjust former handling of a cut off stmt.
	(vect_analyze_slp_instance): Adjust.
	(vect_analyze_slp): Likewise.
	(_bb_vec_info::~_bb_vec_info): Likewise.
	(vectorizable_bb_reduc_epilogue): Dump something if we fail.
	(vect_slp_check_for_constructors): Handle non-internal
	defs as remain defs of a reduction.
	(vectorize_slp_instance_root_stmt): Adjust.

	* gcc.dg/vect/bb-slp-75.c: New testcase.