Replace getOS() == llvm::Triple::*BSD with isOS*BSD() [NFCI]

Replace multiple comparisons of getOS() value with FreeBSD, NetBSD,
OpenBSD and DragonFly with matching isOS*BSD() methods.  This should
improve the consistency of coding style without changing the behavior.
Direct getOS() comparisons were left whenever used in switch or switch-
like context.

Differential Revision: https://reviews.llvm.org/D55916

llvm-svn: 349752