JS: fix error in generation of if/else statements in some cases

See KT-19495

js/js.translator/testData/box/expression/if/ifElseCurlyBraces.kt

@@ -0,0 +1,24 @@
+// EXPECTED_REACHABLE_NODES: 996
+var log = ""
+
+fun foo(s: String, a: Int, b: Int) {
+    when (s) {
+        "A" -> if (a > b) {
+            log += "1"
+        }
+        else if (b > a) {
+            log += "2"
+        }
+        "B" -> log += "3"
+    }
+}
+
+fun box(): String {
+    foo("A", 3, 2)
+    foo("A", 2, 3)
+    foo("B", 2, 3)
+
+    if (log != "123") return "fail: $log"
+
+    return "OK"
+}