RISC-V: Refactor RVV iterators[NFC]

This patch refactors RVV iteratros for easier maintain.

E.g.

(define_mode_iterator V [
  RVVM8QI RVVM4QI RVVM2QI RVVM1QI RVVMF2QI RVVMF4QI (RVVMF8QI "TARGET_MIN_VLEN > 32")

  RVVM8HI RVVM4HI RVVM2HI RVVM1HI RVVMF2HI (RVVMF4HI "TARGET_MIN_VLEN > 32")

  (RVVM8HF "TARGET_VECTOR_ELEN_FP_16") (RVVM4HF "TARGET_VECTOR_ELEN_FP_16") (RVVM2HF "TARGET_VECTOR_ELEN_FP_16")
  (RVVM1HF "TARGET_VECTOR_ELEN_FP_16") (RVVMF2HF "TARGET_VECTOR_ELEN_FP_16")
  (RVVMF4HF "TARGET_VECTOR_ELEN_FP_16 && TARGET_MIN_VLEN > 32")

  RVVM8SI RVVM4SI RVVM2SI RVVM1SI (RVVMF2SI "TARGET_MIN_VLEN > 32")

  (RVVM8SF "TARGET_VECTOR_ELEN_FP_32") (RVVM4SF "TARGET_VECTOR_ELEN_FP_32") (RVVM2SF "TARGET_VECTOR_ELEN_FP_32")
  (RVVM1SF "TARGET_VECTOR_ELEN_FP_32") (RVVMF2SF "TARGET_VECTOR_ELEN_FP_32 && TARGET_MIN_VLEN > 32")

  (RVVM8DI "TARGET_VECTOR_ELEN_64") (RVVM4DI "TARGET_VECTOR_ELEN_64")
  (RVVM2DI "TARGET_VECTOR_ELEN_64") (RVVM1DI "TARGET_VECTOR_ELEN_64")

  (RVVM8DF "TARGET_VECTOR_ELEN_FP_64") (RVVM4DF "TARGET_VECTOR_ELEN_FP_64")
  (RVVM2DF "TARGET_VECTOR_ELEN_FP_64") (RVVM1DF "TARGET_VECTOR_ELEN_FP_64")
])

change it into:

(define_mode_iterator V [VI VF_ZVFHMIN])

gcc/ChangeLog:

	* config/riscv/vector-iterators.md: Refactor iterators.