Add fast path to analyzing special form assignments (#16561)

This showed up as hot spot in a CPU profile collected when running
tests.

This makes `mypy/test/testcheck.py` about 2% faster on my Linux desktop.

mypy/semanal.py

@@ -2855,22 +2855,23 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
         if self.check_and_set_up_type_alias(s):
             s.is_alias_def = True
             special_form = True
-        # * type variable definition
-        elif self.process_typevar_declaration(s):
-            special_form = True
-        elif self.process_paramspec_declaration(s):
-            special_form = True
-        elif self.process_typevartuple_declaration(s):
-            special_form = True
-        # * type constructors
-        elif self.analyze_namedtuple_assign(s):
-            special_form = True
-        elif self.analyze_typeddict_assign(s):
-            special_form = True
-        elif self.newtype_analyzer.process_newtype_declaration(s):
-            special_form = True
-        elif self.analyze_enum_assign(s):
-            special_form = True
+        elif isinstance(s.rvalue, CallExpr):
+            # * type variable definition
+            if self.process_typevar_declaration(s):
+                special_form = True
+            elif self.process_paramspec_declaration(s):
+                special_form = True
+            elif self.process_typevartuple_declaration(s):
+                special_form = True
+            # * type constructors
+            elif self.analyze_namedtuple_assign(s):
+                special_form = True
+            elif self.analyze_typeddict_assign(s):
+                special_form = True
+            elif self.newtype_analyzer.process_newtype_declaration(s):
+                special_form = True
+            elif self.analyze_enum_assign(s):
+                special_form = True
 
         if special_form:
             self.record_special_form_lvalue(s)