[OpenMP] Use AAAssumptionInfo to get assumptions in OpenMPOpt

This patch uses the abstract attributor introduced in D111054 to get the
assumption values instead of the `hasAssumption` function. This also
calls it so assumption information should propagate throug the device
where applicabile.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D111445