[LoongArch] Fix codegen of atomicrmw nand

Fix invalid RISCV-like MI being emitted for performing the `not`
operation: the LoongArch `xori` zero-extends the immediate, hence is
not equivalent to RISCV `xori`. The LoongArch `not` is a `nor` with
zero.

Differential Revision: https://reviews.llvm.org/D136021