[Unifomity] Fix type of ValueRefNull (NFC)

ValueRefNull has no current users.

Author: perlfu

llvm/include/llvm/ADT/GenericSSAContext.h

@@ -54,7 +54,7 @@ template <typename _FunctionT> class GenericSSAContext {
 
   // The null value for ValueRefT. For LLVM IR and MIR, this is simply the
   // default constructed value.
-  static constexpr ValueRefT *ValueRefNull = {};
+  static constexpr ValueRefT ValueRefNull = {};
 
   // An InstructionT usually defines one or more ValueT objects.
   using InstructionT = typename SSATraits::InstructionT;