[ValueTracking] Add comment that CannotBeOrderedLessThanZero does the wrong thing for powi.

Summary:
CannotBeOrderedLessThanZero(powi(x, exp)) returns true if
CannotBeOrderedLessThanZero(x).  But powi(-0, exp) is negative if exp is
odd, so we actually want to return SignBitMustBeZero(x).

Except that also isn't right, because we want to return true if x is
NaN, even if x has a negative sign bit.

What we really need in order to fix this is a consistent approach in
this function to handling the sign bit of NaNs.  Without this it's very
difficult to say what the correct behavior here is.

Reviewers: hfinkel, efriedma, sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28927

llvm-svn: 293243