Properly check expression nullability on attempt to apply == optimizations

#Fix KT-19767

compiler/testData/codegen/box/boxingOptimization/kt19767.kt

@@ -0,0 +1,21 @@
+fun foo(p: Int?): Boolean {
+    return M<Int>(p)?.nulled() == 1
+}
+
+fun foo2(p: Int?): Boolean {
+    return 1 == M<Int>(p)?.nulled()
+}
+
+class M<T: Any>(val z: T?) {
+    fun nulled(): T? = z
+}
+
+
+fun box(): String {
+    if (foo(null)) return "fail 1"
+    if (!foo(1)) return "fail 2"
+
+    if (foo2(null)) return "fail 1"
+    if (!foo2(1)) return "fail 2"
+    return "OK"
+}