Cache argument of when expression (should be evaluated only once).

js/js.translator/testFiles/patternMatching/cases/whenEvaluatesArgumentOnlyOnce.kt

@@ -0,0 +1,12 @@
+package foo
+
+fun box() : Boolean {
+  var a = 0
+  var i = 0
+  when(i++) {
+    -100 -> a++
+    100 -> a++
+    else -> a++
+  }
+  return (a == 1) && (i == 1)
+}