Teach RaiseControlFlow to handle IfOp's with partially infered shapes,

inserting shape_casts as necessary.

Along the way:
 - Add some missing accessors to the AtLeastNOperands trait.
 - Implement shape_cast / ShapeCastOp standard op.
 - Improve handling of errors in mlir-opt, making it easier to understand
   errors when invalid IR is rejected by the verifier.

PiperOrigin-RevId: 211897877