The quite common empty object literal doesn't need an AllocationSite
since it starts off with the general ElementsKind. By using a separate 
bytecode we can directly instantiate the empty object without jumping
to the runtime first.

Note: this experimentally disables pretenuring for empty object
      literals. Depending on the outcome of our benchmarks pretenuring
      will be enabled again or fully removed for empty object literals.

Bug: v8:6211
Change-Id: I2fee81cbefc70865fc436dbd3bc5fc8de04db91c
Reviewed-on: https://chromium-review.googlesource.com/577555
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47467}