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

[Jonathan Thackray via llvm-commits]

https://github.com/jthackray approved this pull request.

LGTM, new testcase added for the specific issue mentioned in the bug report.

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