Fix a self-memcpy which only breaks under Valgrind's memcpy
implementation. Silliness, but it'll be a trivial performance
optimization. This should clear up a failure on the vg_leak bot.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195704 91177308-0d34-0410-b5e6-96231b3b80d8