Add None constants.

pytest/Compiler/constants.py

@@ -76,3 +76,11 @@ def ellipsis():
   # CHECK: basicpy.unknown_cast %[[A]]
   a = ...
   return a
+
+# CHECK-LABEL: func @none_constant
+@import_global
+def none_constant():
+  # CHECK: %[[A:.*]] = basicpy.singleton : !basicpy.NoneType
+  # CHECK: basicpy.unknown_cast %[[A]]
+  a = None
+  return a

python/npcomp/compiler/frontend.py

@@ -191,6 +191,8 @@ class ExpressionImporter(BaseNodeVisitor):
       self.value = ir_h.basicpy_bool_constant_op(True).result
     elif value is False:
       self.value = ir_h.basicpy_bool_constant_op(False).result
+    elif value is None:
+      self.value = ir_h.basicpy_singleton_op(ir_h.basicpy_NoneType).result
     elif isinstance(value, int):
       # TODO: Configurable type mapping
       ir_type = ir_h.i64_type