[OpenMP] Make default parallel for schedule in NVPTX target regions in SPMD mode achieve coalescing

Summary: Set default schedule for parallel for loops to schedule(static, 1) when using SPMD mode on the NVPTX device offloading toolchain to ensure coalescing.

Reviewers: ABataev, Hahnfeld, caomhin

Reviewed By: ABataev

Subscribers: jholewinski, guansong, cfe-commits

Differential Revision: https://reviews.llvm.org/D52629

llvm-svn: 343260