[LICM/mustexec] Extend first iteration must execute logic to fcmps

This builds on the work from https://reviews.llvm.org/D44287. It turned out supporting fcmp was much easier than I realized, so let's do that now.

As an aside, our -O3 handling of a floating point IVs leaves a lot to be desired. We do convert the float IV to an integer IV, but do so late enough that many other optimizations are missed (e.g. we don't vectorize).

Differential Revision: https://reviews.llvm.org/D44542

llvm-svn: 327722