[IR] Reimplement FPMathOperator::classof as a whitelist.
This makes it much easier to verify that the implementation matches the
documentation. It uncovered a bug in the unit tests where we were
accidentally setting fast math flags on a load instruction.
Reviewers: spatel, wristow, arsenm, hfinkel, aemerson, efriedma, cameron.mcinally, mcberg2017, jmolloy
Subscribers: wdng, llvm-commits
Differential Revision: https://reviews.llvm.org/D69176