InstCombine: Fold and/or of fcmp into class

This is motivated by patterns like !isfinite || zero. The AMDGPU math
libraries have a lot of patterns like this, and I'm trying to fix the
code to be more portable and less dependent on directly calling class
intrinsics.

I believe this is the first place where new is.fpclass calls are
introduced. There are more class-like compares that could be
recognized; this is a set I currently care about plus a few extras.

Keep the == 0 cases disabled for now. It depends on the denormal
mode. If we just check IEEE mode now, it will break my use case
without another patch I'm working on.