This lets us avoid allocating the "this" variable for every
generator, since the BytecodeGenerator can directly read
the receiver via BytecodeArrayBuilder::Receive() when passing
it into %_CreateJSGeneratorObject.

Bug: v8:6351
Change-Id: Ib5e1f3303b6b5d5fc051ce76ea62129fd6afac65
Reviewed-on: https://chromium-review.googlesource.com/500507
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#45281}