[llvm] [AArch64][CostModel] Add SVE bfloat arithmetic tests (NFC) (PR #166951)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

When adding the fp16 tests for fp/neon (which is in a similar situation to bf16 for sve), we separate the fp16 tests into their own function and then add two runs lines with and without +fullfp16. Can we do the same here so we can test both costs?

https://github.com/llvm/llvm-project/pull/166951