Refactor: Simplify boolean conditional return statements in lib/Target/NVPTX

Summary: Use clang-tidy to simplify boolean conditional return statements

Reviewers: rafael, echristo, chandlerc, bkramer, craig.topper, dexonsmith, chapuni, eliben, jingyue, jholewinski

Subscribers: llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D9983

llvm-svn: 243734