[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

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@348521 91177308-0d34-0410-b5e6-96231b3b80d8