[CUDA, MemCpyOpt] Add a flag to force-enable memcpyopt and use it for CUDA.

Attempt to enable MemCpyOpt unconditionally in D104801 uncovered the fact that
there are users that do not expect LLVM to materialize `memset` intrinsic.

While other passes can do that, too, MemCpyOpt triggers it more frequently and
breaks sanitizers and some downstream users.

For now introduce a flag to force-enable the flag and opt-in only CUDA
compilation with NVPTX back-end.

Differential Revision: https://reviews.llvm.org/D106401