[llvm] [AMDGPU] GCNHazardRecognizer: refactor getWaitStatesSince (NFCI) (PR #108347)

[Carl Ritson via llvm-commits]

perlfu wrote:

> The logic looks good to me. I just added some nits and musings inline.

Thanks.
I still need to add a test.

A follow up is to flatten `hasHazard` and convert some of the current uses of `getWaitStatesSince` to `hasHazard`.
This is because they don't actually need the wait state count, so shouldn't be incurring the cost of this search.

https://github.com/llvm/llvm-project/pull/108347