This will make it easier if we want to split it into two intrinsics, one
for creating an object with `done == true` and one with `done == false`.

Also remove apparently-dead method FullCodegen::EmitCreateIteratorResult.

Bug: v8:6408, v8:6409
Change-Id: I3d6022a9eff517dd8b664d65950502c22447b364
Reviewed-on: https://chromium-review.googlesource.com/543567Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46107}