The AsyncGeneratorYield builtin just invoked the
AsyncGeneratorResolve() stub anyways, so this removes the middle-man.

Really minor refactoring, but clears out a bit of snapshot size and
another context index.

BUG=v8:5855
R=rmcilroy@chromium.org, bmeurer@chromium.org

Change-Id: I3385a5c5412e8d58493601874c2ad6b60e613012
Reviewed-on: https://chromium-review.googlesource.com/471913
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44820}