[JS IR] IC improvements

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

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

@@ -1,2 +1,5 @@
 
-fun box() = foo0()
+fun box(): String {
+    if (foo0() != 42) return "Fail"
+    return "OK"
+}

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

@@ -2,9 +2,9 @@ MODULES: lib1, main
 
 STEP 0:
     libs: lib1, main
-
+    dirty js: stdlib, lib1, main
 STEP 1:
     libs: lib1, main
-
+    dirty js: lib1
 STEP 2:
     libs: lib1, main