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

[Matt Arsenault via llvm-commits]

================
@@ -1413,10 +1425,16 @@ bool WaitcntGeneratorGFX12Plus::applyPreexistingWaitcnt(
   MachineInstr *CombinedStoreDsCntInstr = nullptr;
   MachineInstr *WaitInstrs[NUM_EXTENDED_INST_CNTS] = {};
 
+  LLVM_DEBUG(dbgs() << "GFX12Plus::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