[OpenMP][NVPTX] Enable default scheduling for parallel for in non-SPMD cases.

Summary: This patch enables the choosing of the default schedule for parallel for loops even in non-SPMD cases.

Reviewers: ABataev, caomhin

Reviewed By: ABataev

Subscribers: jholewinski, guansong, cfe-commits

Differential Revision: https://reviews.llvm.org/D53443

llvm-svn: 345507