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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

Note, this fixes a regression introduced in https://github.com/llvm/llvm-project/pull/83436 brought up here:
https://github.com/llvm/llvm-project/pull/83436#issuecomment-2006090921

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