FIR: intersect flexible type bounds separately

^KT-54522 Fixed

compiler/testData/diagnostics/testsWithStdLib/java/intersectFlexibleAndMutable.kt

@@ -0,0 +1,14 @@
+// FIR_IDENTICAL
+// FILE: Util.java
+import java.util.List;
+
+public class Util {
+    public static <T> List<T> id(List<T> x) { return x; }
+}
+
+// FILE: main.kt
+fun main() {
+    var list = mutableListOf(1)
+    list = Util.id(list)
+    list += 2
+}