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

[Jonathan Thackray via llvm-commits]

jthackray wrote:

> @jthackray Just pushed a change that achieves the same thing a much cleaner way. I'm much happier with this change!

Yes, good improvement, and removes the error string change to the other testcase.

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