Fix confusion about load_p in vect_build_slp_tree_1

Fix confusion about load_p in vect_build_slp_tree_1

load_p is set and used as to whether the stmt is a memory operation,
not whether it is only a load.  The following renames it to ldst_p
to avoid this confusion.  It also replaces checking for a VUSE
with checking STMT_VINFO_DATA_REF since VUSE checking doesn't
work for pattern matched stores where no virtual operands are
present.  Where we want to distinguish between loads and stores
we then check DR_IS_READ/WRITE.

I've made a classification mistake with .MASK_STORE support and
this hits other complications when dealing with single-lane SLP.

	* tree-vect-slp.cc (vect_build_slp_tree_1): Rename
	load_p to ldst_p, fix mistakes and rely on
	STMT_VINFO_DATA_REF.