This should be the last code backout for this. merging this branch
should get us back to the way we were (+ additional code changes for
later changes) as fasr as the unused unboxed code is concerned.
This simplifies the code a bit because
ElementAccessHasExtraIndexedProperty checks for length-overflow
(directly) and sparse-indexes (through TypeCanHaveExtraIndexedProperties)
so the callers don't have to do that anymore.