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

[Alfie Richards via llvm-commits]

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@@ -235,10 +235,10 @@
 @ CHECK-ERRORS: error: immediate value out of range
 @ CHECK-ERRORS:         pkhtb r2, r2, r3, asr #33
 @ CHECK-ERRORS:                                ^
-@ CHECK-ERRORS: error: lsl operand expected.
+@ CHECK-ERRORS: error: shift operator is malformed for this instruction
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AlfieRichardsArm wrote:

Yeah, the old mechanism for the diagnostic was throwing an error when parsing failed.
Unfortunately with the optional operand change the parsing gets applied to more operands and is expected to return NoMatch when there genuinely isn't a match. 

Actually, I still handle this better. Let me fix this with a better diagnostic really quickly.

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