Don't use std::copy and std::copy_backward, run 10% faster.

Don't use std::copy and std::copy_backward, run 10% faster.

Sometimes std::copy can become a memmove call, and that is not a good idea when
copying relatively few bytes as we are doing. We also get a small win by
changing two loops into one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120265 91177308-0d34-0410-b5e6-96231b3b80d8