[llvm] [AMDGPU][NFC] dump Waitcnt using an ostream operator (PR #171251)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

> Not actually used? Should this be marked with LLVM_DUMP_METHOD?

I looked at other instance of ostream operators, but couldn't find any that are marked as LLVM_DUMP_METHOD. As far as I understood, that attribute is meant for actual functions that might be called from a debugger, and hence should not be stripped from a debug build even if they appear to be unused. That doesn't seem necessary for this operator. It's okay if it gets stripped if it is unused.

Separately, I added a use for the operator anyway. More uses will show up with other changes in si-insert-waitcnts later.

https://github.com/llvm/llvm-project/pull/171251