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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> 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.

I'm not sure it's worthwhile, it'll result in two PRs with a similar amount of churn (both needing about the same level of review -- checking the before/after expressions in `framelayout-sve.mir`).

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