Box/unbox nullable inline class values with null check

When we have a nullable inline class value with non-null underlying
type, corresponding value in unboxed representation is nullable. E.g.:

  inline class Str(val value: String)

  fun test(s: Str?) = listOf(s)

Here 'test(s: Str?)' accepts nullable 'java.lang.String' as a parameter.

When boxing/unboxing nullable values of such inline classes, take care
of nulls.

 #KT-26052 Fixed Target versions 1.3-M2

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

@@ -0,0 +1,46 @@
+// !LANGUAGE: +InlineClasses
+// WITH_RUNTIME
+// IGNORE_BACKEND: JVM_IR, JS_IR
+
+inline class X(val x: String)
+inline class Y(val y: Number)
+
+inline class NX(val x: String?)
+inline class NY(val y: Number?)
+
+fun testNotNull(x: X?, y: Y?) {
+    val xs = listOf<Any?>(x)
+    val ys = listOf<Any?>(y)
+    if (!xs.contains(y)) throw AssertionError()
+    if (xs[0] != ys[0]) throw AssertionError()
+    if (xs[0] !== ys[0]) throw AssertionError()
+}
+
+fun testNullable(x: NX?, y: NY?) {
+    val xs = listOf<Any?>(x)
+    val ys = listOf<Any?>(y)
+    if (xs.contains(y)) throw AssertionError()
+    if (xs[0] == ys[0]) throw AssertionError()
+    if (xs[0] === ys[0]) throw AssertionError()
+}
+
+fun testNullsAsNullable(x: NX?, y: NY?) {
+    val xs = listOf<Any?>(x)
+    val ys = listOf<Any?>(y)
+    if (!xs.contains(y)) throw AssertionError()
+    if (xs[0] != ys[0]) throw AssertionError()
+    if (xs[0] !== ys[0]) throw AssertionError()
+}
+
+
+fun box(): String {
+    testNotNull(null, null)
+
+    testNullable(NX(null), NY(null))
+    testNullable(NX(null), null)
+    testNullable(null, NY(null))
+
+    testNullsAsNullable(null, null)
+
+    return "OK"
+}