[llvm] [AArch64][SVE] Remove isSVECC() in favour of changing the calling convention (PR #152742)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Going to close this PR, while I think currently this is valid, I think the main confusing thing about `isSVECC` is the name (since it implies it changes/is the calling convention), whereas what it means is that we augment the calling convention if we have SVE args/results.  Having this as a separate bit of state would be needed if we were ever to support "preserve_allcc" (and others) with SVE, though currently these less well-tested combinations are broken, e.g., see: https://godbolt.org/z/MdbsMMxeh

https://github.com/llvm/llvm-project/pull/152742