[mlir] Small touchups to LangRef attribute section

- attribute-dict production is redundant with dictionary-attribute
- definitions of attribute aliases were part of the same production as
  uses of attribute aliases
- `std.dim` now accepts the dimension number as an operand, so the
  example is out of date. Use the predicate of std.cmpi as a better
  example.

Differential Revision: https://reviews.llvm.org/D96076