[JS] Refactor and comment for typeOf fix

compiler/testData/codegen/box/reflection/typeOf/js/typeOfReifiedUnit.kt

@@ -5,8 +5,9 @@
 
 import kotlin.reflect.typeOf
 
+var x: Int = 1
 fun sideEffects() {
-    println("Side effect")
+    x++
 }
 
 inline fun <reified T> foo(): String {

js/js.inliner/src/org/jetbrains/kotlin/js/inline/clean/substituteKTypes.kt

@@ -26,7 +26,10 @@ fun substituteKTypes(root: JsNode) {
     visitor.accept(root)
 }
 
-// Get KType from expression or from staticRef of referenced name
+// Get KType from expression or from staticRef of referenced name.
+// kType metadata is set on jsClass expressions.
+// There can be a chain of local variables from jsClass to its usage.
+// This methods uses staticRef to find the original expression with kType metadata.
 fun getTransitiveKType(e: JsExpression): JsExpression? {
     return when {
         e.kType != null ->