[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?
> 
> The slt instruction requires both parameters to be sign extended.

Sure. So I sign-extend the `StoreVal`. And `Incr` has been sign-extend, since it is passed by a register: ABI requires it's sign-extended.

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