[llvm] [ARM] FIX: change `pkhtb` custom parsing produce NoMatch rather than Error (PR #85765)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo commented:

+1, thanks, this does fix my build issue. I can't say I fully understand the code changes here (without spending another 15 minutes on reading up on what's going on here), but it does unbreak things.

I'm leaving it unapproved still, if someone else from Arm wants to have a look and give it a proper review, otherwise I guess this can go in later today.

https://github.com/llvm/llvm-project/pull/85765