[PATCH] RISC-V: Support group size of three in SLP store permute lowering

The following implements the group-size three scheme from vect_permute_store_chain in SLP grouped store permute lowering and extends it to power-of-two multiples of group size three. The scheme goes from vectors A, B and C to { A[0], B[0], C[0], A[1], B[1], C[1], ... } by first producing { A[0], B

[PATCH] RISC-V: Support group size of three in SLP store permute lowering

The following implements the group-size three scheme from vect_permute_store_chain in SLP grouped store permute lowering and extends it to power-of-two multiples of group size three. The scheme goes from vectors A, B and C to { A[0], B[0], C[0], A[1], B[1], C[1], ... } by first producing { A[0], B