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

[YunQiang Su via llvm-commits]

wzssyqa wrote:

Just for reference: there is 
```
sllv    $7, $5, $10
...
seh     $7, $7
```

Maybe they are not needed at all. The ABI requires the arguments passed in registers to be well sign-extended.

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