Optimize subtype checking by avoiding a nested function (#14325)

Mypyc isn't good at compiling nested functions, and this one was in one
of the hottest code paths in all of mypy. The nested function wasn't
even used that often, but mypyc would still construct a closure object
every time.

This adds some code duplication, but it's well worth it. Amazingly, this
speeds up self-check by about 10%, if my measurements are to be trusted!

This addresses some of the slowdown introduced in #13303. #14324
addresses another related slowdown.

mypy/subtypes.py

@@ -289,18 +289,20 @@ def _is_subtype(
         # ErasedType as we do for non-proper subtyping.
         return True
 
-    def check_item(left: Type, right: Type, subtype_context: SubtypeContext) -> bool:
-        if proper_subtype:
-            return is_proper_subtype(left, right, subtype_context=subtype_context)
-        return is_subtype(left, right, subtype_context=subtype_context)
-
     if isinstance(right, UnionType) and not isinstance(left, UnionType):
         # Normally, when 'left' is not itself a union, the only way
         # 'left' can be a subtype of the union 'right' is if it is a
         # subtype of one of the items making up the union.
-        is_subtype_of_item = any(
-            check_item(orig_left, item, subtype_context) for item in right.items
-        )
+        if proper_subtype:
+            is_subtype_of_item = any(
+                is_proper_subtype(orig_left, item, subtype_context=subtype_context)
+                for item in right.items
+            )
+        else:
+            is_subtype_of_item = any(
+                is_subtype(orig_left, item, subtype_context=subtype_context)
+                for item in right.items
+            )
         # Recombine rhs literal types, to make an enum type a subtype
         # of a union of all enum items as literal types. Only do it if
         # the previous check didn't succeed, since recombining can be
@@ -312,9 +314,16 @@ def check_item(left: Type, right: Type, subtype_context: SubtypeContext) -> bool
             and (left.type.is_enum or left.type.fullname == "builtins.bool")
         ):
             right = UnionType(mypy.typeops.try_contracting_literals_in_union(right.items))
-            is_subtype_of_item = any(
-                check_item(orig_left, item, subtype_context) for item in right.items
-            )
+            if proper_subtype:
+                is_subtype_of_item = any(
+                    is_proper_subtype(orig_left, item, subtype_context=subtype_context)
+                    for item in right.items
+                )
+            else:
+                is_subtype_of_item = any(
+                    is_subtype(orig_left, item, subtype_context=subtype_context)
+                    for item in right.items
+                )
         # However, if 'left' is a type variable T, T might also have
         # an upper bound which is itself a union. This case will be
         # handled below by the SubtypeVisitor. We have to check both