[llvm] [LoongArch][NFC] Fix the operand constraint of AMCAS instructions (PR #114508)

[WÁNG Xuěruì via llvm-commits]

xen0n wrote:

> AMADD_B, AMXOR__DB_D, AMXOR_DB_W escape from the check.
> 
> From the inc file, it should be
> 
> ```c++
> Opc >= LoongArch::AMADD_B && Opc <= LoongArch::AMXOR_DB_W
> ```

Then it's a functional change. As I already plan to fix the root cause of the problem once and for all in #114398 I'm not going to temporarily fix it here.

https://github.com/llvm/llvm-project/pull/114508