Propagate KotlinType into when expression codegen

This commit removes unneeded boxing when result expression of `when` is
 value of inline class type

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

@@ -0,0 +1,18 @@
+// !LANGUAGE: +InlineClasses
+
+inline class Foo<T>(val x: Any) {
+    fun bar() {}
+}
+
+fun <T, K> transform(f: Foo<T>): Foo<K> {
+    return when {
+        true -> f as Foo<K>
+        else -> TODO()
+    }
+}
+
+fun box(): String {
+    val f = Foo<Int>(42)
+    val t = transform<Int, Number>(f)
+    return if (t.x !is Number) "Fail" else "OK"
+}