[mlir] Use PyValue instead of PyOpResult in Python operand container

The PyOpOperands container was erroneously constructing objects for
individual operands as PyOpResult. Operands in fact are just values,
which may or may not be results of another operation. The code would
eventually crash if the operand was a block argument. Add a test that
exercises the behavior that previously led to crashes.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D90917