[llvm] [AArch64][SME] Use `mrs %reg, SVCR` rather than ABI routine with `+sme` (PR #156878)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm commented:

Is there still any test remaining that covers streaming-compatible -> non-streaming without `+sme`?

https://github.com/llvm/llvm-project/pull/156878