[SystemZ] Use MVC for memcpy

Use MVC for memcpy in cases where a single MVC is enough.  Using MVC is
a win for longer copies too, but I'll leave that for later.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185802 91177308-0d34-0410-b5e6-96231b3b80d8
[SystemZ] Remove unwanted part from last commit

I was originally going to use MVC for memmove too, but that's less of
a clear win.  Remove some accidental left-overs in the previous commit.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185804 91177308-0d34-0410-b5e6-96231b3b80d8