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

[Sergei Barannikov via llvm-commits]

================
@@ -2019,7 +2058,7 @@ emitConvertFuncs(CodeGenTarget &Target, StringRef ClassName,
   CvtOS
       << "                              std::begin(TiedAsmOperandTable)) &&\n";
   CvtOS << "             \"Tied operand not found\");\n";
-  CvtOS << "      unsigned TiedResOpnd = TiedAsmOperandTable[OpIdx][0];\n";
+  CvtOS << "      unsigned TiedResOpnd = TiedAsmOperandTable[*(p + 1)][0];\n";
----------------
s-barannikov wrote:

> I cant imagine a circumstance where it generates an OOB access though?

OOB access can happen in HasOptionalOperands case if `*(p + 1)` (index into TiedAsmOperandTable) is larger than the size of DefaultsOffset array.


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