JS: optimize variable representation in coroutines

Don't convert local variables to fields of coroutine object
when variable is both used and defined in a single block.

js/js.translator/testData/box/coroutines/localVarOptimization.kt

@@ -0,0 +1,49 @@
+// EXPECTED_REACHABLE_NODES: 1124
+// DECLARES_VARIABLE: function=doResume name=k
+// PROPERTY_READ_COUNT: name=local$o count=1
+// PROPERTY_WRITE_COUNT: name=local$o count=2
+import kotlin.coroutines.experimental.*
+
+var next: () -> Unit = {}
+var complete = false
+var log = ""
+
+suspend fun foo(x: String): String = suspendCoroutine { continuation ->
+    log += "[$x]"
+    next = { continuation.resume(x) }
+}
+
+fun build(x: suspend () -> Unit) {
+    next = {
+        x.startCoroutine(object : Continuation<Unit> {
+            override val context = EmptyCoroutineContext
+
+            override fun resume(x: Unit) {
+                complete = true
+            }
+
+            override fun resumeWithException(x: Throwable) {
+                complete = true
+            }
+        })
+    }
+}
+
+fun box(): String {
+    build {
+        val o = foo("O")
+        log += "-"
+        val k = foo("K")
+        log += ":"
+        log += "{$o$k}"
+    }
+
+    while (!complete) {
+        next()
+        log += "#"
+    }
+
+    if (log != "[O]#-[K]#:{OK}#") return "fail: $log"
+
+    return "OK"
+}