[SystemZ] Improve spilling of LGDR and LDGR

If the source of these instructions is spilled we should load the destination.
If the destination is spilled we should store the source.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186147 91177308-0d34-0410-b5e6-96231b3b80d8