[OpenMPOpt] initial tests for ICV tracking. Only nthreads is used.

Summary: Couple of tests to showcase what will be done and what to expect with ICV tracking.

Reviewers: jdoerfert, JonChesterfield

Subscribers: yaxunl, guansong, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81114