[MemCpyOpt] Check for throwing calls during call slot optimization

When performing call slot optimization for a non-local destination,
we need to check whether there may be throwing calls between the
call and the copy. Otherwise, the early write to the destination
may be observable by the caller.

This was already done for call slot optimization of load/store,
but not for memcpys. For the sake of clarity, I'm moving this check
into the common optimization function, even if that does need an
additional instruction scan for the load/store case.

As efriedma pointed out, this check is not sufficient due to
potential accesses from another thread. This case is left as a TODO.

Differential Revision: https://reviews.llvm.org/D88799