Since index_vector is lowered into step_vector in D100816, we can just remove
index_vector, use step_vector for codegen directly.
Please can you also remove INDEX_VECTOR from the AArch64ISD enum.
As a tidy up I'd say these multiclasses are intrinsically linked to stepvector and related nodes. As these nodes now have a unified form I personally don't see much value with the indirection and think they can use all the operations directly within the multiclass.
The add(mul(step_vector comments in this class and sve_int_index_rr are missing a close ) for the mul.