[OPENMP] Resolve lost LoopTripCnt for subsequent loops in same thread.

Remove loopTripCnt from threaded device stack after consuming it.
Added a libomptarget DP message to aid in future debugging and to
validate the added testcase, which only runs in Debug build.

Differential Revision: https://reviews.llvm.org/D64808

llvm-svn: 366349