[SystemZ] Use CLC and IPM to implement memcmp

For now this is restricted to fixed-length comparisons with a length
in the range [1, 256], as for memcpy() and MVC.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188163 91177308-0d34-0410-b5e6-96231b3b80d8