[llvm] [AMDGPU][SIMemoryLegalizer] Ensure LDS -> DMA hazards respect fences (PR #220288)

[Pierre van Houtryve via llvm-commits]

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Pierre-vh wrote:

This name is very confusing. I'd rather have the previous inst be renamed into something like `S_waitcnt_dma_storecnt` and `S_waitcnt_dma_dscnt` for the new one.

We initially started with soft waits as a direct counterpart to normal waits but now they're devolving into becoming overloaded pseudo-instructions. 

https://github.com/llvm/llvm-project/pull/220288