[Vectorize] Fix a warning

This patch fixes:

  llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/Legality.h:85:16:
  error: private field 'Reason' is not used
  [-Werror,-Wunused-private-field]

Author: kazutakahirata

llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/Legality.h

@@ -82,7 +82,7 @@ class LegalityResult {
 
 /// Base class for results with reason.
 class LegalityResultWithReason : public LegalityResult {
-  ResultReason Reason;
+  [[maybe_unused]] ResultReason Reason;
   LegalityResultWithReason(LegalityResultID ID, ResultReason Reason)
       : LegalityResult(ID), Reason(Reason) {}
   friend class Pack; // For constructor.