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

[WÁNG Xuěruì via llvm-commits]

xen0n wrote:

Turns out the TableGen change actually caused the test to fail... perfect show of how fragile the original implementation was.

 I've minimally fixed it and the CI should be green now.

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