Apply suggestion from @james-d-mitchell

If I understand correctly, InDegrees is faster than InDegreeOfVertexNC here due to caching behaviour where the in degrees of the digraph are computed once then looked up from cache, as opposed to the other function which has to recompute for each input vertex.

Co-authored-by: James Mitchell <james-d-mitchell@users.noreply.github.com>

Author: ThatOtherAndrew

gap/oper.gi

@@ -2733,7 +2733,7 @@ function(D, v)
   fi;
   hasse := DigraphReflexiveTransitiveReduction(DigraphMutableCopyIfMutable(D));
   # join-irreducible iff at most one lower cover in the Hasse diagram
-  return InDegreeOfVertexNC(hasse, v) <= 1;
+  return InDegrees(hasse)[v] <= 1;
 end);
 
 InstallMethod(IsMeetIrreducible,