[llvm] [SystemZ][z/OS] Remove use of subsections. (PR #184167)

[Kai Nacke via llvm-commits]

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redstar wrote:

I broke up `FrameAndFPROffset` and `FrameAndVROffset`. I did not change the `*Mask` members, because those are basically a bitset representation of a set of bits.

https://github.com/llvm/llvm-project/pull/184167