[llvm] (reland) [AMDGPU][SIInsertWaitCnts] Use RegUnits-based tracking (#162077) (PR #171779)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

Added a testcase reduced from the Blender source, and in which the waitcnts are affected if the merge is not done properly.

https://github.com/llvm/llvm-project/pull/171779