generate more correct bytecode for casting Unit to other types

compiler/testData/codegen/box/casts/asAny.kt

@@ -0,0 +1,7 @@
+fun println(s: String) {
+}
+
+fun box(): String {
+    println(":Hi!") as Any
+    return "OK"
+}

compiler/testData/codegen/box/casts/asUnit.kt

@@ -0,0 +1 @@
+fun box() = if (4 as? Unit != null) "Fail" else "OK"

compiler/testData/codegen/box/casts/lambdaToUnitCast.kt

@@ -0,0 +1,6 @@
+val foo: () -> Unit = {}
+
+fun box(): String {
+    foo() as Unit
+    return "OK"
+}