Fix relative execution location introspection (#5741)

So, I was reading the new Flow type strictness and noticed
https://flow.org/blog/2017/05/07/Strict-Function-Call-Arity/
Specifically, I wondered whether the `sum_all` example would copy the
arguments into an array, then loop over. Sadly, it does.

```js
function sum_all(...rest) {
  let ret = 0;
  for (let i = 0; i < rest.length; i++) { ret += rest[i]; }
  return ret;
}

// output
function sum_all() {
  var ret = 0;
  for (var _len = arguments.length, rest = Array(_len), _key = 0; _key < _len; _key++) {
    rest[_key] = arguments[_key];
  }
  for (var i = 0; i < rest.length; i++) { ret += rest[i]; }
  return ret;
}
```

But then I noticed if I changed `let i = 0` to `let i: number = 0`, it
worked directly on `arguments`. That lead me down a rabbit hole to
`Path#_guessExecutionStatusRelativeTo`. When tracing through, the last
comparison made no sense to me. It was trying to find the index of
`"init"` in a list of `["declarations"]` and `"body"` in `["directives",
"body"]`. Red flags and such.

But it makes sense when you're trying to compare the visitor order of
the common ancestor path. Then we're trying to find `"init"` in a list
of `["init", "test", "update", "body"]`. Oh, and there's `"body"` in
there too! And now we know the `ForStatement`'s `init` is executed
before the `body`.

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-member-expression-optimisation/expected.js

@@ -20,11 +20,7 @@ function t() {
 
 
 function t() {
-  for (var _len = arguments.length, items = Array(_len), _key = 0; _key < _len; _key++) {
-    items[_key] = arguments[_key];
-  }
-
-  for (var i = 0; i < items.length; i++) {
-    return items[i];
+  for (var i = 0; i < arguments.length; i++) {
+    return arguments.length <= i ? undefined : arguments[i];
   }
 }