Support boxing/unboxing for method return types

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

@@ -0,0 +1,35 @@
+// !LANGUAGE: +InlineClasses
+
+inline class Foo(val a: Int) {
+    fun member(): String = ""
+
+    fun asResult() = a
+}
+
+fun <T> id(x: T): T = x
+fun <T> T.idExtension(): T = this
+
+fun Foo.extension() {}
+
+
+fun test(f: Foo): String {
+    id(f) // box
+    id(f).idExtension() // box
+
+    id(f).member() // box unbox
+    id(f).extension() // box unbox
+
+    val a = id(f) // box unbox
+    val b = id(f).idExtension() // box unbox
+
+    if (a.asResult() != 10) return "fail"
+    if (b.asResult() != 10) return "fail"
+
+    return "OK"
+}
+
+fun box(): String {
+    val f = Foo(10)
+
+    return test(f)
+}

compiler/testData/codegen/bytecodeText/inlineClasses/boxUnboxInlineClassFromMethodReturnType.kt

@@ -0,0 +1,28 @@
+// !LANGUAGE: +InlineClasses
+
+inline class Foo(val a: Int) {
+    fun member(): String = ""
+}
+
+fun <T> id(x: T): T = x
+fun <T> T.idExtension(): T = this
+
+fun Foo.extension() {}
+
+
+fun test(f: Foo) {
+    id(f) // box
+    id(f).idExtension() // box
+
+    id(f).member() // box unbox
+    id(f).extension() // box unbox
+
+    val a = id(f) // box unbox
+    val b = id(f).idExtension() // box unbox
+}
+
+// 6 INVOKESTATIC Foo\$Erased.box
+// 4 INVOKEVIRTUAL Foo.unbox
+
+// 0 valueOf
+// 0 intValue

compiler/testData/codegen/bytecodeText/inlineClasses/unboxInlineClassesAfterSmartCasts.kt

@@ -0,0 +1,28 @@
+// !LANGUAGE: +InlineClasses
+
+inline class Foo(val x: Int) {
+    fun member() {}
+}
+
+fun Foo?.extensionOnNullable() {}
+
+fun test(f: Foo?) {
+    if (f != null) {
+        f.member() // unbox
+    }
+
+    if (f != null) {
+        f.extensionOnNullable()
+    }
+
+    if (f != null) {
+        val a = f
+        a.member() // unbox
+    }
+}
+
+// 0 INVOKESTATIC Foo\$Erased.box
+// 2 INVOKEVIRTUAL Foo.unbox
+
+// 0 valueOf
+// 0 intValue