[mlir][arith] Further clean up select op definition (#93358)

* Improve the condition type requirement description ('scalar' ->
signless i1), to match what is actually verified.
* Use the `I1` type predicate instead of `AnyBooleanTypeMatch`.

Related discussion:
https://github.com/llvm/llvm-project/pull/93351#issuecomment-2130453233.