Revert "[NodeTypeResolver] Apply logic Mixed inside Union check on TypeFactor…" (#7347)

This reverts commit dd60a7c.

Author: samsonasik

src/NodeTypeResolver/PHPStan/Type/TypeFactory.php

@@ -64,10 +64,8 @@ public function uniquateTypes(array $types, bool $keepConstant = false): array
         $hasFalse = false;
         $hasTrue = false;
         foreach ($types as $type) {
-            // mixed only allowed to be standalone
             if ($type instanceof MixedType) {
-                /** @var array<TType> */
-                return [new MixedType()];
+                $type = new MixedType();
             }
 
             $type = $this->normalizeObjectType($totalTypes, $type);
@@ -170,6 +168,12 @@ private function createUnionOrSingleType(array $types): Type
             return $types[0];
         }
 
+        foreach ($types as $type) {
+            if ($type instanceof MixedType) {
+                return new MixedType();
+            }
+        }
+
         return new UnionType($types);
     }