[JS IR] IC improvements

- Reuse a built fragment
  - Use cache for saving built JS code

js/js.translator/testData/incremental/invalidation/suspendFunctions/main/m.kt

@@ -1,6 +1,8 @@
 
-suspend fun box(): Int {
+suspend fun suspendBox(): Int {
     val x = fooX()
     val y = fooY()
     return x + y
 }
+
+fun box() = "OK"

js/js.translator/testData/incremental/invalidation/suspendFunctions/project.info

@@ -2,7 +2,10 @@ MODULES: lib1, main
 
 STEP 0:
     libs: lib1, main
+    dirty js: stdlib, lib1, main
 STEP 1:
     libs: lib1, main
+    dirty js: lib1, main
 STEP 2:
     libs: lib1, main
+    dirty js: stdlib, lib1, main