refactor: Compute work from headers without CBlockIndex

Avoid the need to construct a CBlockIndex object just to compute work for a header,
when its nBits value suffices for that.

Co-Authored-By: Pieter Wuille <pieter@wuille.net>

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