[llvm] [IR] Remove the possibility of ConstantExpr having fast-math flags. (PR #94507)

[Joshua Cranmer via llvm-commits]

jcranmer-intel wrote:

> It would be good to also make the class inherit from Instruction instead of Operator (in this context, "Operator" means "Instruction or ConstantExpr").

I've considered that as well. `FPMathOperator` was originally an `Operator` when it was first implemented and `Instruction`-only, so I've reverted it back to that form for now. I've been poking around having FMF be present on `Instruction` in lieu of `SubclassOptionalData`, at which case the inheritance from `Instruction` is far more helpful. If that is the route taken to add more FMF bits, then I'll do it as part of that change.

https://github.com/llvm/llvm-project/pull/94507