[mlir][DictionaryAttr] Add a new getWithSorted and use it when possible

The elements of a DictionaryAttr are sorted by name. In many situations, e.g NamedAttributeList, we can guarantee that the elements are sorted on construction and remove the need to perform extra checks. In places with lots of calls to attribute methods, this leads to a good performance improvement.

Differential Revision: https://reviews.llvm.org/D78781