Add more tests for inline class equality

compiler/testData/codegen/box/inlineClasses/equalityChecksMixedNullability.kt

@@ -0,0 +1,14 @@
+// !LANGUAGE: +InlineClasses
+
+inline class A(val a: String)
+
+fun isEqualNA(x: A?, y: A) = x == y
+fun isEqualAN(x: A, y: A?) = x == y
+
+fun box(): String {
+    if (isEqualNA(null, A(""))) return "Fail 1"
+    if (isEqualAN(A(""), null)) return "Fail 2"
+    if (!isEqualNA(A(""), A(""))) return "Fail 3"
+    if (!isEqualAN(A(""), A(""))) return "Fail 4"
+    return "OK"
+}