[mlir][vector] Simplify createReadOrMaskedRead (#163736)
Simplify `createReadOrMaskedRead` to only require _one_ argument to
specify the vector type to read (passed as `VectorType`) instead of
passing vector-sizes and scalable-flags independently (i.e. _two_
arguments).
A simple overload is provided for users that wouldn't re-use the
corresponding `VectorType` (and hence there's no point for them
to create). While there are no users upstream for this overload,
it's been helpful downstream.
