Place the LoadField for the backing store of the [[IteratedObject]]
before the diamond to make it possible to eliminate this LoadField
in LoadElimination later, when used in `for..of` or destructing.

This further improves the performance of `for..of` in the micro
benchmark on the tracking bug from

  console.timeEnd: forOf, 191.726000
  console.timeEnd: traditional, 107.572000
  console.timeEnd: forOf, 137.288000
  console.timeEnd: traditional, 102.976000
  console.timeEnd: forOf, 137.506000
  console.timeEnd: traditional, 103.089000

to around

  console.timeEnd: forOf, 195.238000
  console.timeEnd: traditional, 107.078000
  console.timeEnd: forOf, 128.980000
  console.timeEnd: traditional, 103.106000
  console.timeEnd: forOf, 128.525000
  console.timeEnd: traditional, 103.072000

so roughly another ~7% improvement (with untrusted code mitigations
turned off).

Bug: v8:8070
Change-Id: I34831c503384f0cc44b95317dd84403f2ed8ecd5
Reviewed-on: https://chromium-review.googlesource.com/1188138Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55390}