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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I went ahead and landed this now, to unbreak my builds. I tried to update the commit message according to the solution chosen in the end. (If unhappy with the way I phrased it, feel free to revert and recommit with a more correct message.)

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