[llvm] [IR][CodeGen] Specify llvm.gcroot contents are an opaque zero-initialized blob (PR #222333)

[via llvm-commits]

================
@@ -148,11 +149,13 @@ static bool CouldBecomeSafePoint(Instruction *I) {
       isa<LoadInst>(I))
     return false;
 
-  // llvm.gcroot is safe because it doesn't do anything at runtime.
+  // llvm.gcroot is safe because it doesn't do anything at runtime.  A memset
----------------
isuckatcs wrote:

```suggestion
  // llvm.gcroot is safe because it doesn't do anything at runtime. A memset
```

https://github.com/llvm/llvm-project/pull/222333