[llvm] [LLVM][SVE] Honour calling convention when using SVE for fixed length vectors. (PR #70847)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

- Rebased.
- Finalised the support by matching the behaviour for types that should result in scalarisation.
- Added tests.

https://github.com/llvm/llvm-project/pull/70847