[llvm] [AArch64][SVE] Tweak how SVE CFI expressions are emitted (PR #151677)

[Gaëtan Bossu via llvm-commits]

https://github.com/gbossu commented:

Looks sensible to me, I'm just not comfortable approving (yet?) as I haven't checked all the computations in detail.

Super-nit: It would be even clearer to have a pure NFC PR with the refactorings, followed by another one with the optimisation for small constants.

https://github.com/llvm/llvm-project/pull/151677