bugfix: only allow narrowing down types in case the old type is not single

Signed-off-by: Maximilian Bösing <2189546+boesing@users.noreply.github.com>

src/Psalm/Internal/Analyzer/Statements/Expression/CallAnalyzer.php

@@ -1139,6 +1139,6 @@ class CallAnalyzer
             return true;
         }
 
-        return $new_type->allLiterals();
+        return !$old_type->isSingle();
     }
 }