[OPENMP][NVPTX]Revert __kmpc_shuffle_int64 to its original form.

Summary:
Use the original shuffle implementation for __kmpc_shuffle_int64 since
default implementation uses the same implementation.

Reviewers: gtbercea

Subscribers: guansong, caomhin, openmp-commits

Differential Revision: https://reviews.llvm.org/D55514

llvm-svn: 348772