[llvm] IR: Add attribute negated (PR #121027)

[YunQiang Su via llvm-commits]

wzssyqa wrote:

> Using attributes for this is not appropriate. This should probably be a separate intrinsic (if anything).

Since we have 4 cases here, use aarch64 as an example:
```
fmadd: c + a *b
fnmadd: (-c) + (-a) * b
fmsub: c - a * b
fnmsub: -c - (-a) * b
```

In the backend, `fneg` can merged with `fmuladd` on platform that support FMA so we have `fmuladd` only.
If we introduce more intrinsics, we will have lots of backend work.
    

https://github.com/llvm/llvm-project/pull/121027