Remove overly verbose debug from GenericDomTreeConstruction

[mehdi_amini]

Summary

This particular debug shows all the time when I'm processing any MLIR
module:

discovered a new reachable node ^bb0
discovered a new reachable node ^bb0
discovered a new reachable node ^bb0
discovered a new reachable node ^bb0

...
(repeated x1875)

I think that printing all the basic blocks in the function is likely low
value enough that we can get away with removing this.

llvm/include/llvm/Support/GenericDomTreeConstruction.h

struct SemiNCAInfo {
   }
 
   void attachNewSubtree(DomTreeT& DT, const TreeNodePtr AttachTo) {
     // Attach the first unreachable block to AttachTo.
     NodeToInfo[NumToNode[1]].IDom = AttachTo->getBlock();
     // Loop over all of the discovered blocks in the function...
     for (size_t i = 1, e = NumToNode.size(); i != e; ++i) {
       NodePtr W = NumToNode[i];
-      LLVM_DEBUG(dbgs() << "\tdiscovered a new reachable node "
-                        << BlockNamePrinter(W) << "\n");
 
       // Don't replace this with 'count', the insertion side effect is important
       if (DT.DomTreeNodes[W]) continue;  // Haven't calculated this node yet?
 
       NodePtr ImmDom = getIDom(W);
 
       // Get or calculate the node for the immediate dominator.
       TreeNodePtr IDomNode = getNodeForBlock(ImmDom, DT);