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

[YunQiang Su via llvm-commits]

wzssyqa wrote:

[test.c.gz](https://github.com/llvm/llvm-project/files/15057837/test.c.gz)

I test with this C code.

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