[OpenMP] Avoid remainder operations for loop index values on a collapsed loop nest.

Summary: Change the strategy for computing loop index variables after collapsing a loop nest via the collapse clause by replacing the expensive remainder operation with multiplications and additions.

Reviewers: ABataev, caomhin

Reviewed By: ABataev

Subscribers: guansong, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D56413

llvm-svn: 350759