Avoid division by zero via constant_multiple_p

With recent SLP vectorization patches I see RISC-V divison by zero
for gfortran.dg/matmul_10.f90 and others in get_group_load_store_type
which does

              && can_div_trunc_p (group_size
                                  * LOOP_VINFO_VECT_FACTOR (loop_vinfo) - gap,
                                  nunits, &tem, &remain)
              && (known_eq (remain, 0u)
                  || (constant_multiple_p (nunits, remain, &num)
                      && (vector_vector_composition_type (vectype, num,
                                                          &half_vtype)
                          != NULL_TREE))))
            overrun_p = false;

where for [2, 2] / [0, 2] the condition doesn't reflect what we
are trying to test - that, when remain is zero or, when non-zero,
nunits is a multiple of remain, we can avoid touching a gap via
loading smaller pieces and vector composition.

It isn't safe to change the known_eq to maybe_eq so instead
require known_ne (remain, 0u) before doing constant_multiple_p.
There's the corresponding code in vectorizable_load that's known
to have a latent similar issue, so sync that up as well.

	* tree-vect-stmts.cc (get_group_load_store_type): Check
	known_ne (remain, 0u) before doing constant_multiple_p.
	(vectorizable_load): Likewise.

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