XFAIL a failing SystemZ test (llvm#1043)

This test fails in amd-gfx because we are carrying a revert of
10ed1ec "[MachineSink] Allow sinking of constant or ignorable
physreg uses".

This did not affect PSDB/OSDB testing because we do not normally enable
the SystemZ target.

Author: jayfoad

llvm/test/CodeGen/SystemZ/machinesink-dead-cc.mir

@@ -6,6 +6,8 @@
 #
 # Test that the AGHIK can be sunk into %bb.4. It has a def of CC, but it is dead.
 
+# XFAIL: *
+
 --- |
   define void @fun() {  ret void  }
 ...