[AArch64] NFC: Clarify and auto-generate some CodeGen tests.

* For ext-narrow-index.ll, move vscale_range attribute closer to the
  function definition, rather than through indirect #<num> attribute. This
  makes the test a bit easier to read.
* auto-generated CHECK lines for sve-cmp-select.ll and
  named-vector-shuffles-sve.ll.
* re-generated CHECK lines for tests that had a mention they were
  auto-generated, but where the CHECK lines were out of date.