[llvm] [SystemZ][z/OS] Reverse the order of instructions to save and restore CSRs (PR #179540)

[via llvm-commits]

sujianIBM wrote:

> It seems odd to define them in inverted order and then invert them again for output. Is there a reason for that?

When the sequence is processed in `assignCalleeSavedSpillSlots()`, the FrameIdx of each register is decided. The register processed first gets the smallest idx, which leads to the highest address on stack (XPLINK). So at this point we want `F15` to be processed first and put on the highest address.
In the following `spillCalleeSavedRegisters()` and `restoreCalleeSavedRegisters()`, we want the instruction on `F15` to be the last.

https://github.com/llvm/llvm-project/pull/179540