aarch64; Add support for vector offset ranges

Some SME2 instructions operate on a range of consecutive ZA vectors.
This is indicated by syntax such as:

   za[<Wv>, <imml>:<immh>]

Like with the earlier vgx2 and vgx4 support, we get better error
messages if the parser allows all ZA indices to have a range.
We can then reject invalid cases during constraint checking.