[JS IR] IC improvements

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

js/js.translator/testData/incremental/invalidation/fastPath2/main/m.kt

@@ -1,2 +1,2 @@
 
-fun box() = qux()
+fun box() = if (qux() == 42) "OK" else "Fail"

js/js.translator/testData/incremental/invalidation/fastPath2/main/module.info

@@ -1,7 +1,6 @@
 STEP 0:
     dependencies: stdlib, lib1, lib2
     dirty: m.kt
-
 STEP 1:
     dependencies: stdlib, lib1, lib2
-    flags: FP
+    flags: FP