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

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

The first commit of this patch is the untouched original patch. The second commit has the changes from the initial review.

I did not add a new testcase, I want to but this issue is super difficult to reduce. It happens only in Blender cycles (and apparently only on one scene!). None of our lit tests, other llvm-test-suite projects, or internal test suite projects have any issues with this.

I'm still trying to get a testcase but I am going on holidays soon so I'm not sure I'll have one in time. I'd like to land this without the added test case if possible (the buildbot will still flag any issue, even if it's not ideal) and I'll put a PR up for a test separately if I finally get a reduced version.

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