[OPENMP]Fix overflow during counting the number of iterations.

Summary:
The OpenMP loops are normalized and transformed into the loops from 0 to
max number of iterations. In some cases, original scheme may lead to
overflow during calculation of number of iterations. If it is unknown,
if we can end up with overflow or not (the bounds are not constant and
  we cannot define if there is an overflow), cast original type to the
  unsigned.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, sstefan1, openmp-commits, cfe-commits, caomhin

Tags: #clang, #openmp

Differential Revision: https://reviews.llvm.org/D81881