[mlir] fix Operation::getDiscardableAttrs in absence of properties (#76816)

When properties are not enabled in an operation, inherent attributes are
stored in the common dictionary with discardable attributes. However,
`getDiscardableAttrs` and `getDiscardableAttrDictionary` were returning
the entire dictionary, making the caller mistakenly believe that all
inherent attributes are discardable. Fix this by filtering out
attributes whose names are registered with the operation, i.e., inherent
attributes. This requires an API change so `getDiscardableAttrs` returns
a filter range.