[AArch64][SVE] NFC: Use stepvector directly in index multiclasses

Also fixes a couple of warnings identified in D109359:

  SVEInstrFormats.td:5099:59: warning: unused template argument: sve_int_index_ri::step_vector
  SVEInstrFormats.td:5133:59: warning: unused template argument: sve_int_index_rr::step_vector

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D109422