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

This possibility was added in https://reviews.llvm.org/D34303 to resolve
some assertion failures with cases where FP math operations got
constant-folded in constant expressions. However, at no point did the IR
representation allow for expressing fast-math flags on constant
expressions.

With the change of https://github.com/llvm/llvm-project/pull/93038,
there are no longer any constant expressions capable of being FP math
operators, and thus FPMathOperator can go back to being
Instruction-only.