[mlir][Python] Add `__hash__` implementation for Block.

[mikeurbach]

Summary

This allows us to hash Blocks and use them in sets or parts of larger
hashable objects. The implementation is the same as other core IR
constructs: the C API object's pointer is hashed.

[mikeurbach]

I think this change may be trivial enough to land without pre-merge approval. I've been testing it in a downstream project and it seems to work as expected.

mlir/lib/Bindings/Python/IRCore.cpp

py::class_<PyBlock>(m, "Block", py::module_local())
                                        firstOperation);
           },
           "Iterates over operations in the block.")
       .def("__eq__",
            [](PyBlock &self, PyBlock &other) {
              return self.get().ptr == other.get().ptr;
            })
       .def("__eq__", [](PyBlock &self, py::object &other) { return false; })
+      .def("__hash__",
+           [](PyBlock &self) {
+             return static_cast<size_t>(llvm::hash_value(self.get().ptr));
+           })
       .def(
           "__str__",
           [](PyBlock &self) {
             self.checkValid();
             PyPrintAccumulator printAccum;
             mlirBlockPrint(self.get(), printAccum.getCallback(),
                            printAccum.getUserData());
             return printAccum.join();

mlir/test/python/ir/blocks.py

with InsertionPoint(module.body):
       with InsertionPoint(block):
         ret_op = Operation.create("ret", operands=[block.arguments[0]])
       realop = Operation.create("realop",
                                 results=[r.type for r in ret_op.operands],
                                 regions=1)
       block.append_to(realop.operation.regions[0])
       dummy.operation.erase()
     print(module)
+
+
+# CHECK-LABEL: TEST: testBlockHash
+@run
+def testBlockHash():
+  with Context() as ctx, Location.unknown():
+    ctx.allow_unregistered_dialects = True
+    module = Module.create()
+    f32 = F32Type.get()
+    with InsertionPoint(module.body):
+      dummy = Operation.create("dummy", regions=1)
+      block1 = Block.create_at_start(dummy.operation.regions[0], [f32])
+      block2 = Block.create_at_start(dummy.operation.regions[0], [f32])
+      assert hash(block1) != hash(block2)