[FPEnv][InstSimplify] 0.0 - -X ==> X

Another ticket split out of D107285, this extends the optimization
of 0.0 - -X to just X when using constrained intrinsics and the
optimization is allowed.

If the negation of X is done with fsub then the match fails because of
the lack of IR Matcher support for constrained intrinsics.

While I'm here, remove some TODO notices since the work is no longer
planned.

Differential Revision: https://reviews.llvm.org/D131607