There are some accesses that we know that are pointers, and we can
specialize them to have Pointer as their Machine Type and Write Barrier.

Drive-by fix: ForJSGeneratorObjectParametersAndRegisters can be changed
to use Pointer.

Bug: v8:7703
Change-Id: I88e371746fc5f08c84795c95b1885264ef6c067c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1741658
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63223}