[mlir] Remove tabs from predecessor comments

[GMNGeoffrey]

Summary

This change removes tabs from the comments printed by the asmprinter after basic
block declarations in favor of two spaces. This is currently the only place in
the printed IR that uses tabs.

[jpienaar]

Yay no tabs

[GMNGeoffrey]

Can someone land this for me, since I don't have commit access?

[GMNGeoffrey]

Thanks Mehdi :-)

mlir/lib/IR/AsmPrinter.cpp

if (!block->args_empty()) {
         printType(arg.getType());
       });
       os << ')';
     }
     os << ':';
 
     // Print out some context information about the predecessors of this block.
     if (!block->getParent()) {
-      os << "\t// block is not in a region!";
+      os << "  // block is not in a region!";
     } else if (block->hasNoPredecessors()) {
-      os << "\t// no predecessors";
+      os << "  // no predecessors";
     } else if (auto *pred = block->getSinglePredecessor()) {
-      os << "\t// pred: ";
+      os << "  // pred: ";
       printBlockName(pred);
     } else {
       // We want to print the predecessors in increasing numeric order, not in
       // whatever order the use-list is in, so gather and sort them.
       SmallVector<std::pair<unsigned, Block *>, 4> predIDs;
       for (auto *pred : block->getPredecessors())
         predIDs.push_back({state->getSSANameState().getBlockID(pred), pred});
       llvm::array_pod_sort(predIDs.begin(), predIDs.end());
 
-      os << "\t// " << predIDs.size() << " preds: ";
+      os << "  // " << predIDs.size() << " preds: ";
 
       interleaveComma(predIDs, [&](std::pair<unsigned, Block *> pred) {
         printBlockName(pred.second);
       });
     }
     os << newLine;
   }