Boxing values merge fix. #KT-5588 Fixed

When merging Boxed value and something else just return MIXED_VALUE

compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt5588.kt

@@ -0,0 +1,10 @@
+fun box() : String {
+    val s = "notA"
+    val id = when (s) {
+        "a" -> 1
+        else -> null
+    }
+
+    if (id == null) return "OK"
+    return "fail"
+}