Let the strcat optimizer return the pointer to the start of the buffer,

instead of the place where it started to perform the string copy.

- PR3661
- Patch by Benjamin Kramer!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68443 91177308-0d34-0410-b5e6-96231b3b80d8