[llvm] [AArch64] Refactor redundant PTEST optimisations (NFC) (PR #87802)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm approved this pull request.

I prefer `std::nullopt` over `{}` but I see both styles used so will not worry about it. 

https://github.com/llvm/llvm-project/pull/87802