[llvm] [AMDGPU][SIInsertWaitCnts] Use RegUnits-based tracking (PR #162077)

[Jay Foad via llvm-commits]

jayfoad wrote:

> One issue I've had with that is that I'm not sure how to tell if a RU is a SGPR or VGPR.

You can get the unit's "root" register and test that. See MCRegUnitRootIterator. AMDGPU does not use ad hoc aliasing so I think every unit should have exactly one root.

https://github.com/llvm/llvm-project/pull/162077