[JS IR] IC improvements

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

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

@@ -1,2 +1,10 @@
 
-fun box() = dex(4, 2)
+fun box(stepId: Int): String {
+    when (stepId) {
+        0 -> if (dex(4, 2) != 11) return "Fail"
+        1 -> if (dex(4, 2) != 10) return "Fail"
+        2 -> if (dex(4, 2) != 9) return "Fail"
+        else -> return "Unknown"
+    }
+    return "OK"
+}

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

@@ -1,11 +1,3 @@
-STEP 0:
+STEP 0..2:
     dependencies: stdlib, lib1, lib2, lib3
     dirty: m.kt
-
-STEP 1:
-    dependencies: stdlib, lib1, lib2, lib3
-    dirty: m.kt
-
-STEP 2:
-    dependencies: stdlib, lib1, lib2, lib3
-    dirty: m.kt