[LoongArch] Fix atomic instructions operands sequence

According to https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#atomic-memory-access-instructions,
the operands sequence of am* instructions should be "rd, rk, rj"
but not "rd, rj, rk". Sorry for this typo in initial patches.

Reviewed By: xen0n, MaskRay

Differential Revision: https://reviews.llvm.org/D120107