[JS IR] IC improvements

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

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

@@ -1,2 +1,6 @@
 
-fun box() = qux0() + qux1()
+fun box(): String {
+    if (qux0() != 42) return "Fail"
+    if (qux1() != 24) return "Fail"
+    return "OK"
+}

js/js.translator/testData/incremental/invalidation/removeFile/main/m.kt.new

@@ -1,2 +1,5 @@
 
-fun box() = qux0()
+fun box(): String {
+    if (qux0() != 42) return "Fail"
+    return "OK"
+}

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

@@ -1,9 +1,8 @@
 STEP 0:
     dependencies: stdlib, lib1, lib2
     dirty: m.kt
-
 STEP 1:
     dependencies: stdlib, lib1, lib2
     modifications:
         U : m.kt.new -> m.kt
-    dirty: m.kt
+    dirty: m.kt