[llvm] [AMDGPU] Fix simplification of VM_VSRC based on VMEM waits (PR #178711)

[Ryan Mitchell via llvm-commits]

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@@ -1295,12 +1297,27 @@ void WaitcntBrackets::simplifyVmVsrc(const AMDGPU::Waitcnt &CheckWait,
   // Waiting for some counters implies waiting for VM_VSRC, since an
   // instruction that decrements a counter on completion would have
   // decremented VM_VSRC once its VGPR operands had been read.
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RyanRio wrote:

Can you update the comment to describe the new approach/maybe mention the nuance here?

https://github.com/llvm/llvm-project/pull/178711