[LoongArch] Fix pattern for FNMSUB_{S/D} instructions (#73742)

```
when a=c=-0.0, b=0.0:
-(a * b + (-c)) = -0.0
-a * b + c = 0.0
(fneg (fma a, b (-c))) != (fma (fneg a), b ,c)
```

See https://reviews.llvm.org/D90901 for a similar discussion on X86.