[llvm] [NFC][AMDGPU] print more info when debugging InsertWaitCnts pass (PR #144629)

[Matt Arsenault via llvm-commits]

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@@ -1265,10 +1272,15 @@ bool WaitcntGeneratorPreGFX12::applyPreexistingWaitcnt(
   MachineInstr *WaitcntInstr = nullptr;
   MachineInstr *WaitcntVsCntInstr = nullptr;
 
+  LLVM_DEBUG(dbgs() << "PreGFX12::applyPreexistingWaitcnt at: " << *It << "\n");
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arsenm wrote:

```suggestion
  LLVM_DEBUG(dbgs() << "PreGFX12::applyPreexistingWaitcnt at: " << *It << '\n');
```

Single characters should print as character and not string 

https://github.com/llvm/llvm-project/pull/144629