[LoongArch] Optimize conditional selection of integer

This patch optimizes code generation by leveraging the zeroing behavior of the `maskeqz`/`masknez` instructions.

```
int sel(int a, int b)
{
    return (a < b) ? a : 0;
}
```

```
slt	$a1,$a0,$a1
masknez	$a2,$r0,$a1
maskeqz	$a0,$a0,$a1
or	$a0,$a0,$a2
```

=>

```
slt	$a1,$a0,$a1
maskeqz	$a0,$a0,$a1
```

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D153193