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

[via llvm-commits]

yingopq 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.

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