[llvm] [AMDGPU] introduce S_WAITCNT_LDS_DIRECT in the memory legalizer (PR #150887)

[Pierre van Houtryve via llvm-commits]

https://github.com/Pierre-vh approved this pull request.

Small nit otherwise this LGTM. Name is ok for me, I'd prefer if there was a "_pseudo" at the end instead (maybe `S_WAITCNT_LDS_DIRECT_pseudo`, just to make it clear it isn't a HW inst in any ISA, but it's really bikeshedding at this point

https://github.com/llvm/llvm-project/pull/150887