[IR] Reimplement FPMathOperator::classof as a whitelist.

Summary:
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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69176

llvm-svn: 375252