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

[Matt Arsenault via llvm-commits]

================
@@ -1265,10 +1272,15 @@ bool WaitcntGeneratorPreGFX12::applyPreexistingWaitcnt(
   MachineInstr *WaitcntInstr = nullptr;
   MachineInstr *WaitcntVsCntInstr = nullptr;
 
+  LLVM_DEBUG(dbgs() << "PreGFX12::applyPreexistingWaitcnt at: " << *It << "\n");
+
   for (auto &II :
        make_early_inc_range(make_range(OldWaitcntInstr.getIterator(), It))) {
-    if (II.isMetaInstruction())
+    LLVM_DEBUG(dbgs() << "pre-existing iter: " << II << "\n");
----------------
arsenm wrote:

```suggestion
    LLVM_DEBUG(dbgs() << "pre-existing iter: " << II);
```

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