[APFloat] Make use of new overloaded comparison operators. NFC.

Reviewers: ekatz, spatel, jfb, tlively, craig.topper, RKSimon, nikic, scanon

Subscribers: arsenm, jvesely, nhaehnle, hiraditya, dexonsmith, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75744