[llvm] [msan] Convert target("aarch64.svcount") from compile-time crash to MSan false negatives (PR #165028)

[Florian Mayer via llvm-commits]

================
@@ -1544,6 +1545,27 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     }
   }
 
+  static bool isAArch64SVCount(Type *Ty) {
+    if (!isa<TargetExtType>(Ty))
----------------
fmayer wrote:

Isn't this just:

```cpp
if (TargetExtType* TTy = dyn_cast<TargetExtType>(Ty))
    return TTy->getName() == "aarch64.svcount";
return false;
```

https://github.com/llvm/llvm-project/pull/165028