[OpenMPIRBuilder] Implement collapseLoops.

The collapseLoops method implements a transformations facilitating the implementation of the collapse-clause. It takes a list of loops from a loop nest and reduces it to a single loop that can be used by other methods that are implemented on just a single loop, such as createStaticWorkshareLoop.

This patch shares some changes with D92974 (such as adding some getters to CanonicalLoopNest), used by both patches.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93268