[llvm] [AMDGPU] Update code object metadata for kernarg preload (PR #134666)

[Shilei Tian via llvm-commits]

================
@@ -95,11 +98,79 @@ inline raw_ostream &operator<<(raw_ostream &OS, const ArgDescriptor &Arg) {
   return OS;
 }
 
-struct KernArgPreloadDescriptor : public ArgDescriptor {
-  KernArgPreloadDescriptor() {}
-  SmallVector<MCRegister> Regs;
+namespace KernArgPreload {
+
+enum HiddenArg {
+  HIDDEN_BLOCK_COUNT_X,
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shiltian wrote:

nit:

```suggestion
  HIDDEN_BLOCK_COUNT_X = 0,
```

https://github.com/llvm/llvm-project/pull/134666