[llvm] [KnownFPClass] Add neg_square and fma_neg_square (PR #227248)

[via llvm-commits]

s-mv wrote:

A few inferences I'm noting down here:
- `-x * x` is always -ve/0/nan if not flushed to +0
- `fma(-x, x, y)` never gives a positive result if y is never positive
- `-((-x) * x)` should behave like `x * x` except for the case positive zero denormal case

TODO: tests :melting_face: and more digging around `x - y * y` and corresponding cases

https://github.com/llvm/llvm-project/pull/227248