Support boxing/unboxing for method return types

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