[JS IR] IC improvements

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

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

@@ -1,2 +1,9 @@
 
-fun box() = qux()
+fun box(stepId: Int): String {
+    when (stepId) {
+        0 -> if (qux() != 42) return "Fail"
+        1 -> if (qux() != 99) return "Fail"
+        else -> "Unknown"
+    }
+    return "OK"
+}

js/js.translator/testData/incremental/invalidation/removeModule/main/module.info

@@ -1,6 +1,5 @@
 STEP 0:
     dependencies: stdlib, lib1, lib2
     dirty: m.kt
-
 STEP 1:
     dependencies: stdlib, lib2