Remove custom allocation orders in SystemZ.

Remove custom allocation orders in SystemZ.

Note that this actually changes code generation, and someone who
understands this target better should check the changes.

- R12Q is now allocatable. I think it was omitted from the allocation
  order by mistake since it isn't reserved. It as apparently used as a
  GOT pointer sometimes, and it should probably be reserved if that is
  the case.

- The GR64 registers are allocated in a different order now. The
  register allocator will automatically put the CSRs last. There were
  other changes to the order that may have been significant.

The test fix is because r0 and r1 swapped places in the allocation order.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133067 91177308-0d34-0410-b5e6-96231b3b80d8