[llvm] [AMDGPU] Simplify WaitcntBrackets::getRegInterval with getPhysRegBaseClass (PR #74087)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

You should absolutely try to make use of the MCInstrDesc. Looking up the physical register class is extremely expensive. At one point the waitcnt pass was one of the most expensive backend passes, spending most of its time looking up physical register classes 

https://github.com/llvm/llvm-project/pull/74087