[MemCpyOpt] Enable memcpy optimizations unconditionally.

The patch does not depend on the availability of the library functions for
memcpy/memset as it operates on LLVM intrinsics.  The optimizations are useful
on the targets that have these functions disabled (e.g. NVPTX & AMDGPU).

Differential Revision: https://reviews.llvm.org/D104801