[llvm] [Utils] update_llc_test_checks.py: updated the regexp for ARM target (PR #148287)

[Tommaso Fellegara via llvm-commits]

Felle33 wrote:

The only things that I have modified are:
- To capture the name of the function in AArch64 I had to specify the `[a-zA-Z][a-zA-Z0-9]*`
- the optionality to have the second part of the signature at the beginning of the assembly function `@@<func_name>` for the other regex

I tested these regex with the ARM/ifcvt-iter-indbr.ll, ARM/atomic-cmpxchg.ll and AArch64/arm64-arith.ll

https://github.com/llvm/llvm-project/pull/148287