Interfaces can be optional: whether an op implements an interface or not
can depend on the state of the operation.
```
// An optional code block for adding additional "classof" logic. This can
// be used to better enable "optional" interfaces, where an entity only
// implements the interface if some dynamic characteristic holds.
// `$_attr`/`$_op`/`$_type` may be used to refer to an instance of the
// interface instance being checked.
code extraClassOf = "";
```
The current `Pass::canScheduleOn(RegisteredOperationName)` is
insufficient. This commit adds an additional overload to inspect
`Operation *`.
This commit fixes a crash when scheduling an `InterfacePass` for an
optional interface on an operation that does not actually implement the
interface.
This is a re-upload of #168499, which was reverted.
fe56f5c3d3