Fix casts of Unit value to other types

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

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

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

@@ -0,0 +1,8 @@
+fun println(s: String) {
+}
+
+fun box(): String {
+    val x = println(":Hi!") as Any
+    if (x != Unit) return "Fail: $x"
+    return "OK"
+}

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

@@ -0,0 +1,8 @@
+fun println(s: String) {
+}
+
+fun box(): String {
+    val x = println(":Hi!") as? Any
+    if (x != Unit) return "Fail: $x"
+    return "OK"
+}