[clang] [llvm] [AMDGPU] Introduce asyncmark/wait intrinsics (PR #173259)

[Pierre van Houtryve via cfe-commits]

================
@@ -0,0 +1,179 @@
+===============================
+ AMDGPU Asynchronous Operations
+===============================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+Asynchronous operations are memory transfers (usually between the global memory
+and LDS) that are completed independently at an unspecified scope. A thread that
+requests one or more asynchronous transfers can use *async markers* to track
+their completion. The thread waits for each marker to be *completed*, which
+indicates that requests initiated in program order before this marker have also
+completed.
+
+Operations
+==========
+
+``async_load_to_lds``
+---------------------
+
----------------
Pierre-vh wrote:

```suggestion
Legacy LDS DMA (GFX9-10)
---------------------------------------
```

I wonder if this should be separated in two sections ? e.g.

* Synchronization Operations
  * asyncmark
  * wait.asyncmark(N)
* Memory Operations
  * Legacy LDS DMA (GFX9-10)

https://github.com/llvm/llvm-project/pull/173259