Adjust assert in vect_build_slp_tree_2

The assert in SLP discovery when we handle masked operations is
confusingly wide - all gather variants should be catched by
the earlier STMT_VINFO_GATHER_SCATTER_P.

	* tree-vect-slp.cc (vect_build_slp_tree_2): Only expect
	IFN_MASK_LOAD for masked loads that are not
	STMT_VINFO_GATHER_SCATTER_P.