[MemCpyOpt] Require writable object during call slot optimization (#71542)

Call slot optimization may introduce writes to the destination object
that occur earlier than in the original function. We currently already
check that that the destination is dereferenceable and aligned, but we
do not make sure that it is writable. As such, we might introduce a
write to read-only memory, or introduce a data race.

Fix this by checking that the object is writable. For arguments, this is
indicated by the new writable attribute. Tests using
sret/dereferenceable are updated to use it.