[mlir][python] Fix MemRefType IsAFunction in Python bindings

[ftynse]

Summary

MemRefType was using a wrong isa function in the bindings code, which
could lead to invalid IR being constructed. Also run the verifier in
memref dialect tests.

mlir/lib/Bindings/Python/IRTypes.cpp

public:
   }
 };
 
 class PyMemRefLayoutMapList;
 
 /// Ranked MemRef Type subclass - MemRefType.
 class PyMemRefType : public PyConcreteType<PyMemRefType, PyShapedType> {
 public:
-  static constexpr IsAFunctionTy isaFunction = mlirTypeIsARankedTensor;
+  static constexpr IsAFunctionTy isaFunction = mlirTypeIsAMemRef;
   static constexpr const char *pyClassName = "MemRefType";
   using PyConcreteType::PyConcreteType;
 
   PyMemRefLayoutMapList getLayout();
 
   static void bindDerived(ClassTy &c) {
     c.def_static(
          "get",

mlir/python/mlir/dialects/_memref_ops_ext.py

Args:
       indices: the list of subscripts, may be empty for zero-dimensional
         buffers.
       loc: user-visible location of the operation.
       ip: insertion point.
     """
     memref_resolved = _get_op_result_or_value(memref)
     indices_resolved = [] if indices is None else _get_op_results_or_values(
         indices)
-    return_type = memref_resolved.type
+    return_type = MemRefType(memref_resolved.type).element_type
     super().__init__(return_type, memref, indices_resolved, loc=loc, ip=ip)

mlir/test/python/dialects/memref.py

with Context() as ctx, Location.unknown(ctx):
     func = module.body.operations[0]
     func_body = func.regions[0].blocks[0]
     with InsertionPoint.at_block_terminator(func_body):
       memref.LoadOp(func.arguments[0], func.arguments[1:])
 
     # CHECK: func @f1(%[[ARG0:.*]]: memref<?x?xf32>, %[[ARG1:.*]]: index, %[[ARG2:.*]]: index)
     # CHECK: memref.load %[[ARG0]][%[[ARG1]], %[[ARG2]]]
     print(module)
+    assert module.operation.verify()