refactor: Compute work from headers without CBlockIndex

diff --git a/src/chain.cpp b/src/chain.cpp
index 3dd22634..622e018d 100644
--- a/src/chain.cpp
+++ b/src/chain.cpp
@@ -124,12 +124,12 @@ void CBlockIndex::BuildSkip()
         pskip = pprev->GetAncestor(GetSkipHeight(nHeight));
 }
 
-arith_uint256 GetBlockProof(const CBlockIndex& block)
+arith_uint256 GetBitsProof(uint32_t bits)
 {
     arith_uint256 bnTarget;
     bool fNegative;
     bool fOverflow;
-    bnTarget.SetCompact(block.nBits, &fNegative, &fOverflow);
+    bnTarget.SetCompact(bits, &fNegative, &fOverflow);
     if (fNegative || fOverflow || bnTarget == 0)
         return 0;
     // We need to compute 2**256 / (bnTarget+1), but we can't represent 2**256
diff --git a/src/chain.h b/src/chain.h
index 2c865265..3b8a672d 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -348,7 +348,15 @@ protected:
     CBlockIndex& operator=(CBlockIndex&&) = delete;
 };
 
-arith_uint256 GetBlockProof(const CBlockIndex& block);
+/** Compute how much work an nBits value corresponds to. */
+arith_uint256 GetBitsProof(uint32_t bits);
+
+/** Compute how much work a block index entry corresponds to. */
+inline arith_uint256 GetBlockProof(const CBlockIndex& block) { return GetBitsProof(block.nBits); }
+
+/** Compute how much work a block header corresponds to. */
+inline arith_uint256 GetBlockProof(const CBlockHeader& header) { return GetBitsProof(header.nBits); }
+
 /** Return the time it would take to redo the work difference between from and to, assuming the current hashrate corresponds to the difficulty at tip, in seconds. */
 int64_t GetBlockProofEquivalentTime(const CBlockIndex& to, const CBlockIndex& from, const CBlockIndex& tip, const Consensus::Params&);
 /** Find the forking point between two chain tips. */
diff --git a/src/headerssync.cpp b/src/headerssync.cpp
index e52a8cdc..fc73d403 100644
--- a/src/headerssync.cpp
+++ b/src/headerssync.cpp
@@ -202,7 +202,7 @@ bool HeadersSyncState::ValidateAndProcessSingleHeader(const CBlockHeader& curren
         }
     }
 
-    m_current_chain_work += GetBlockProof(CBlockIndex(current));
+    m_current_chain_work += GetBlockProof(current);
     m_last_header_received = current;
     m_current_height = next_height;
 
@@ -238,7 +238,7 @@ bool HeadersSyncState::ValidateAndStoreRedownloadedHeader(const CBlockHeader& he
     }
 
     // Track work on the redownloaded chain
-    m_redownload_chain_work += GetBlockProof(CBlockIndex(header));
+    m_redownload_chain_work += GetBlockProof(header);
 
     if (m_redownload_chain_work >= m_minimum_required_work) {
         m_process_all_remaining_headers = true;
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 5c8d92f0..5fd11d3c 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -4348,7 +4348,7 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
                 MaybeSendGetHeaders(pfrom, GetLocator(m_chainman.m_best_header), *peer);
             }
             return;
-        } else if (prev_block->nChainWork + CalculateClaimedHeadersWork({{cmpctblock.header}}) < GetAntiDoSWorkThreshold()) {
+        } else if (prev_block->nChainWork + GetBlockProof(cmpctblock.header) < GetAntiDoSWorkThreshold()) {
             // If we get a low-work header in a compact block, we can ignore it.
             LogDebug(BCLog::NET, "Ignoring low-work compact block from peer %d\n", pfrom.GetId());
             return;
@@ -4666,7 +4666,7 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
             mapBlockSource.emplace(hash, std::make_pair(pfrom.GetId(), true));
 
             // Check claimed work on this block against our anti-dos thresholds.
-            if (prev_block && prev_block->nChainWork + CalculateClaimedHeadersWork({{pblock->GetBlockHeader()}}) >= GetAntiDoSWorkThreshold()) {
+            if (prev_block && prev_block->nChainWork + GetBlockProof(pblock->GetBlockHeader()) >= GetAntiDoSWorkThreshold()) {
                 min_pow_checked = true;
             }
         }
diff --git a/src/validation.cpp b/src/validation.cpp
index af523b06..ecc51745 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -4171,8 +4171,7 @@ arith_uint256 CalculateClaimedHeadersWork(std::span<const CBlockHeader> headers)
 {
     arith_uint256 total_work{0};
     for (const CBlockHeader& header : headers) {
-        CBlockIndex dummy(header);
-        total_work += GetBlockProof(dummy);
+        total_work += GetBlockProof(header);
     }
     return total_work;
 }