[clang] [llvm] [AMDGPU] Emit a waitcnt instruction after each memory instruction (PR #79236)

[Jay Foad via cfe-commits]

================
@@ -2326,6 +2326,20 @@ bool SIInsertWaitcnts::insertWaitcntInBlock(MachineFunction &MF,
     }
 #endif
 
+    if (ST->isPreciseMemoryEnabled() && Inst.mayLoadOrStore()) {
+      AMDGPU::Waitcnt Wait;
+      if (ST->hasExtendedWaitCounts())
+        Wait = AMDGPU::Waitcnt(0, 0, 0, 0, 0, 0, 0);
+      else
+        Wait = AMDGPU::Waitcnt(0, 0, 0, 0);
+
+      if (!Inst.mayStore())
+        Wait.StoreCnt = ~0u;
----------------
jayfoad wrote:

```suggestion
      AMDGPU::Waitcnt Wait = WCG->getAllZeroWaitcnt(Inst.mayStore());
```
However, as a general rule:
- loads and atomics-with-return update LOADcnt
- stores and atomics-without-return update STOREcnt

so it might be more accurate to use the condition `Inst.mayStore() && !SIInstrInfo::isAtomicRet(Inst)`.

Please make sure you have tests for atomics with and without return.

https://github.com/llvm/llvm-project/pull/79236