[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