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"
+}

compiler/testData/codegen/box/boxingOptimization/kt19767_2.kt

@@ -0,0 +1,6 @@
+//WITH_RUNTIME
+
+fun box(): String {
+    val map: Map<String, Boolean>? = mapOf()
+    return if (map?.get("") == true) "fail" else "OK"
+}

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"
+}

compiler/testData/codegen/box/boxingOptimization/kt19767_chain.kt

@@ -0,0 +1,23 @@
+fun foo(p: Int?): Boolean {
+    return M<Int>(p)?.chain()?.nulled() == 1
+}
+
+fun foo2(p: Int?): Boolean {
+    return 1 == M<Int>(p)?.chain()?.nulled()
+}
+
+class M<T: Any>(val z: T?) {
+    fun nulled(): T? = z
+
+    fun chain(): M<T>? = this
+}
+
+
+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"
+}