[llvm] [SandboxVectorizer][NFCI] Fix use of possibly-uninitialized scalar. (PR #122201)

[Tyler Lanphear via llvm-commits]

================
@@ -37,10 +38,25 @@ class Context {
   using MoveInstrCallback =
       std::function<void(Instruction *, const BBIterator &)>;
 
-  /// An ID for a registered callback. Used for deregistration. Using a 64-bit
-  /// integer so we don't have to worry about the unlikely case of overflowing
-  /// a 32-bit counter.
-  using CallbackID = uint64_t;
+  /// An ID for a registered callback. Used for deregistration. A dedicated type
+  /// is employed so as to keep IDs opaque to the end user; only Context should
+  /// deal with its underlying representation.
+  class CallbackID {
+  public:
+    // Uses a 64-bit integer so we don't have to worry about the unlikely case
+    // of overflowing a 32-bit counter.
+    using ReprTy = uint64_t;
+
+  private:
+    // Default initialization results in an invalid ID.
+    ReprTy Val = std::numeric_limits<ReprTy>::max();
----------------
tylanphear wrote:

Hmm, this is a good point. I think using 0 as the default value and making the first valid callback ID be 1 makes sense as a solution.

https://github.com/llvm/llvm-project/pull/122201