AMDGPU/GlobalISel: Add stub custom regbankselect pass

Uniformity analysis needs to be the fundamental basis for
regbank decisions. The considerations of the default pass
are secondary, but potentially useful for some edge cases (e.g.
selecting AGPRs when arbitrary loads and stores can directly use
them). This needs to be a separate pass since it requires new
analysis dependencies.

Boilerplate to subclass the existing pass which does nothing
different.