[llvm] [ARM][TableGen][MC] Change the ARM mnemonic operands to be optional for ASM parsing (PR #83436)

[Ivan Kosarev via llvm-commits]

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@@ -1474,7 +1513,7 @@ void AsmMatcherInfo::buildOperandMatchInfo() {
         OperandMask |= maskTrailingOnes<unsigned>(NumOptionalOps + 1)
                        << (i - NumOptionalOps);
       }
-      if (Op.Class->IsOptional)
+      if (Op.Class->IsOptional && Op.Class->OptionalShouldOffsetCustomParsers)
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kosarev wrote:

I struggle to understand why this is necessary. By the look of it, the flag makes the operand effectively non-optional. Can you give an example demonstrating the problem this flag addresses?

https://github.com/llvm/llvm-project/pull/83436