[PatternMatch] m_SpecificMask should pass expected mask by Value. NFC

Unlike m_Mask, we don't need to modify a variable owned by the caller
so we should pass the ArrayRef by value or const reference.

Author: topperc

llvm/include/llvm/IR/PatternMatch.h

@@ -1844,9 +1844,9 @@ struct m_ZeroMask {
 };
 
 struct m_SpecificMask {
-  ArrayRef<int> &MaskRef;
-  m_SpecificMask(ArrayRef<int> &MaskRef) : MaskRef(MaskRef) {}
-  bool match(ArrayRef<int> Mask) { return MaskRef == Mask; }
+  ArrayRef<int> Val;
+  m_SpecificMask(ArrayRef<int> Val) : Val(Val) {}
+  bool match(ArrayRef<int> Mask) { return Val == Mask; }
 };
 
 struct m_SplatOrPoisonMask {