Properly check expression nullability on attempt to apply == optimizations

#Fix KT-19767

compiler/testData/codegen/box/boxingOptimization/kt19767_3.kt

@@ -0,0 +1,34 @@
+// IGNORE_BACKEND: JS, NATIVE
+
+// FILE: M.java
+
+public class M {
+    private final Integer value;
+
+    public M(Integer value) {
+        this.value = value;
+    }
+
+    public Integer nulled() {
+        return value;
+    }
+}
+
+
+// FILE: Kotlin.kt
+fun foo(p: Int?): Boolean {
+    return M(p)?.nulled() == 1
+}
+
+fun foo2(p: Int?): Boolean {
+    return 1 == M(p)?.nulled()
+}
+
+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"
+}