operand: Implement operand hashing and use hashed set for declarations

Instead of manually searching each element in the declarations vector,
use an unordered_set to emplace new declarations avoiding repetition.

src/op.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <cassert>
+#include <climits>
 
 #include "common_types.h"
 #include "literal_number.h"
@@ -47,6 +48,21 @@ bool Op::operator==(const Operand& other) const {
     return false;
 }
 
+std::size_t Op::Hash() const {
+    std::size_t hash = Operand::Hash();
+    hash ^= static_cast<std::size_t>(opcode) << 20;
+    if (result_type) {
+        hash ^= result_type->Hash() << 16;
+    }
+    hash ^= static_cast<std::size_t>(id.value_or(0)) << 8;
+    std::size_t wrap = 32;
+    for (const auto operand : operands) {
+        wrap = (wrap + 7) % (sizeof(std::size_t) * CHAR_BIT);
+        hash ^= operand->Hash() << wrap;
+    }
+    return hash;
+}
+
 void Op::Write(Stream& stream) const {
     stream.Write(static_cast<u16>(opcode));
     stream.Write(WordCount());