[AMDGPU][IGLP] Parameterize the SchedGroup processing / linking order in Solver

Currently the PipelineSolver processes SchedGroups in bottom up manner. However, there is no compelling reason to require this. Providing the option to toggle this affords greater experimentation capability, and make usage a bit more intuitive. Importantly, it makes designing rules much easier.

Differential Revision: https://reviews.llvm.org/D149393

Change-Id: Ic4abd3408f9faa105c0eef72eab7873d46083ee4