[llvm] [LoongArch] Record the special AMO operand constraint with TableGen (PR #114398)

[Xi Ruoyao via llvm-commits]

xry111 wrote:

> The LoongArch Reference Manual says that the 3-register atomic memory operations cannot have their rd equal to either rj or rk [1](#user-content-fn-1-229b5bab4398a513e3058e283dd99671), and both GNU as and LLVM IAS enforce the constraint.

This is not true.  gas 2.43.50.20241031 still accepts `amswap.w $r0, $r0, $r0`.

And I prefer to provide the users a way to raise an INE.

https://github.com/llvm/llvm-project/pull/114398