Deduplicate the struct type

This commit deduplicates IRDifferentialPairUserCodeType types by
ignoring its witness table vale when caching them.

source/slang/slang-ir-autodiff.h

@@ -510,7 +510,33 @@ struct DifferentialPairTypeBuilder
     };
 
     // Cache from pair types to lowered type.
-    Dictionary<IRInst*, IRInst*> pairTypeCache;
+    //
+    // When we cache `IRDifferentialPairUserCodeType`, we should ignore the second operand which is witness table.
+    // If we don't, we will end up emiting duplicated types whose members are identical.
+    // 
+    struct PairCacheKey
+    {
+        IRInst* inst;
+        int depth = 0;
+
+        PairCacheKey(IRInst* i)
+        {
+            while (auto userCode = as<IRDifferentialPairUserCodeType>(i))
+            {
+                i = userCode->getOperand(0);
+                depth++;
+            }
+            inst = i;
+        }
+
+        HashCode getHashCode() const { return combineHash(HashCode(inst), HashCode(depth)); }
+
+        bool operator==(const PairCacheKey& rhs) const
+        {
+            return inst == rhs.inst && depth == rhs.depth;
+        }
+    };
+    Dictionary<PairCacheKey, IRInst*> pairTypeCache;
 
     // Cache from existential pair types to their lowered interface keys.
     // We use a different cache because an interface type can have