refactor: Introduce GetFirstStoredBlock helper function

src/node/blockstorage.cpp

@@ -397,6 +397,16 @@ bool BlockManager::IsBlockPruned(const CBlockIndex* pblockindex)
     return (m_have_pruned && !(pblockindex->nStatus & BLOCK_HAVE_DATA) && pblockindex->nTx > 0);
 }
 
+const CBlockIndex* GetFirstStoredBlock(const CBlockIndex* start_block) {
+    AssertLockHeld(::cs_main);
+    assert(start_block);
+    const CBlockIndex* last_block = start_block;
+    while (last_block->pprev && (last_block->pprev->nStatus & BLOCK_HAVE_DATA)) {
+        last_block = last_block->pprev;
+    }
+    return last_block;
+}
+
 // If we're using -prune with -reindex, then delete block files that will be ignored by the
 // reindex.  Since reindexing works by starting at block file 0 and looping until a blockfile
 // is missing, do the same here to delete any later block files after a gap.  Also delete all