[OPENMP][NVPTX]Correct type casting for printf args + simplified shfl64 function.

Summary:
Explicitly casted printf's args to the required types + simplified
shfl64 function.

Reviewers: gtbercea, kkwli0

Subscribers: guansong, jfb, caomhin, openmp-commits

Differential Revision: https://reviews.llvm.org/D55379

llvm-svn: 348521