Remove STMT_VINFO_VEC_STMTS

Remove STMT_VINFO_VEC_STMTS

The following removes the last uses of STMT_VINFO_VEC_STMTS and
the vector itself.  Vector stmts are recorded in SLP nodes now.

The last use is a bit strange - it was introduced by
Richard S. in r8-6064-ga57776a1136962 and affects only
power7 and below (the re-align optimized load path).  The
check should have never been true since vect_vfa_access_size
is only ever invoked before stmt transform.  I have done
the "conservative" change of making it always true now
(so the code is now entered).  I can as well remove it, but
I wonder if you remember anything about this ...

	* tree-vectorizer.h (_stmt_vec_info::vec_stmts): Remove.
	(STMT_VINFO_VEC_STMTS): Likewise.
	* tree-vectorizer.cc (vec_info::new_stmt_vec_info): Do not
	initialize it.
	(vec_info::free_stmt_vec_info): Nor free it.
	* tree-vect-data-refs.cc (vect_vfa_access_size): Remove
	check on STMT_VINFO_VEC_STMTS.