Fix ConstantExpr opcode to use C++ enum values.

Note: C++ opcode is different from C opcode values.
Add ConstantExpr opcode name.
Add test for these.

test/constexpr.py

@@ -0,0 +1,17 @@
+import unittest
+from llvm.core import *
+
+class TestConstExpr(unittest.TestCase):
+    def test_constexpr_opcode(self):
+        mod = Module.new('test_constexpr_opcode')
+        func = mod.add_function(Type.function(Type.void(), []), name="foo")
+        builder = Builder.new(func.append_basic_block('entry'))
+        a = builder.inttoptr(Constant.int(Type.int(), 123),
+                             Type.pointer(Type.int()))
+        self.assertTrue(isinstance(a, ConstantExpr))
+        self.assertEqual(a.opcode, OPCODE_INTTOPTR)
+        self.assertEqual(a.opcode_name, "inttoptr")
+
+if __name__ == '__main__':
+    unittest.main()
+