[OpenMP] Remove doing assumption propagation in the front end.

This patch removes the assumption propagation that was added in D110655
primarily to get assumption informatino on opaque call sites for
optimizations. The analysis done in D111445 allows us to do this more
intelligently in the back-end.

Depends on D111445

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D111463