[llvm] [MIPS] Sign-extend subwords when expanding atomic max/min (PR #89246)

[YunQiang Su via llvm-commits]

wzssyqa wrote:

[syq-89246.patch](https://github.com/llvm/llvm-project/files/15048858/syq-89246.patch)

@jdmitrovic-syrmia I figure out a patch. I have some test on both big-endian and little endian.
can you have a review?

https://github.com/llvm/llvm-project/pull/89246