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

[YunQiang Su via llvm-commits]

wzssyqa wrote:

And I find that the current code cannot work with big-endian.
```
#include <stdio.h>

short test_max_16 (short *a, short b);

struct xx {
        short a;
        short b;
} s = {0x1234, 0x5678};

int main() {
        short p = test_max_16 (&(s.b), 0x6789);
        printf("%hx, %hx\n", s.b, p);
}
```
```
5678, 5678
```

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