[clang][bytecode] Check memcmp for block pointers (llvm#165070)

We can't read from non-block pointers anyway.

Fixes llvm#165061

Author: tbaederr

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -1941,6 +1941,9 @@ static bool interp__builtin_memcmp(InterpState &S, CodePtr OpPC,
     return true;
   }
 
+  if (!PtrA.isBlockPointer() || !PtrB.isBlockPointer())
+    return false;
+
   bool IsWide =
       (ID == Builtin::BIwmemcmp || ID == Builtin::BI__builtin_wmemcmp);
 

clang/test/AST/ByteCode/builtin-functions.cpp

@@ -1510,6 +1510,8 @@ namespace Memcmp {
   static_assert(f());
 #endif
 
+  int unknown;
+  void foo(void) { unknown *= __builtin_memcmp(0, 0, 2); }
 }
 
 namespace Memchr {