[NFC] Rename minnan and maxnan to minimum and maximum

Summary:
Changes all uses of minnan/maxnan to minimum/maximum
globally. These names emphasize that the semantic difference between
these operations is more than just NaN-propagation.

Reviewers: arsenm, aheejin, dschuff, javed.absar

Subscribers: jholewinski, sdardis, wdng, sbc100, jgravelle-google, jrtc27, atanasyan, llvm-commits

Differential Revision: https://reviews.llvm.org/D53112

llvm-svn: 345218