The register file of the JSGeneratorObject is normally filled with
undefined in the beginning, except in TurboFan where we put the_hole
there. In addition TurboFan used StoreElement to initialize the fields
but then StoreField/LoadField to access them later, which can lead to
aliasing bugs (currently not possible because our alias analysis is
not smart enough).

Bug: v8:7253
Change-Id: Idbff29d138946f110336b9bef0e1889e596d834c
Reviewed-on: https://chromium-review.googlesource.com/952968Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51784}