Minor proper subtype check optimization

Mypyc is bad at compiling nested functions. In one use case
we were spending 7% of the mypy runtime just creating closure
objects for `check_argument`. Here I manually inline the nested
function to avoid this overhead.

Work on #12526.

mypy/subtypes.py

@@ -1365,13 +1365,6 @@ def visit_instance(self, left: Instance) -> bool:
  return True
 
  if left.type.has_base(right.type.fullname):
- def check_argument(leftarg: Type, rightarg: Type, variance: int) -> bool:
- if variance == COVARIANT:
- return self._is_proper_subtype(leftarg, rightarg)
- elif variance == CONTRAVARIANT:
- return self._is_proper_subtype(rightarg, leftarg)
- else:
- return mypy.sametypes.is_same_type(leftarg, rightarg)
  # Map left type to corresponding right instances.
  left = map_instance_to_supertype(left, right.type)
  if self.erase_instances:
@@ -1382,7 +1375,13 @@ def check_argument(leftarg: Type, rightarg: Type, variance: int) -> bool:
  nominal = True
  for ta, ra, tvar in zip(left.args, right.args, right.type.defn.type_vars):
  if isinstance(tvar, TypeVarType):
- nominal = nominal and check_argument(ta, ra, tvar.variance)
+ variance = tvar.variant
+ if variance == COVARIANT:
+ nominal = nominal and self._is_proper_subtype(ta, ra)
+ elif variance == CONTRAVARIANT:
+ nominal = nominal and self._is_proper_subtype(ra, ta)
+ else:
+ nominal = nominal and return mypy.sametypes.is_same_type(ta, ra)
  else:
  nominal = nominal and mypy.sametypes.is_same_type(ta, ra)