[MLIR][NFC] Value print update for block arguments

[bondhugula]

Summary

Emit some more information when printing/dumping Values of
BlockArgument type. This is purely to help for debugging purposes.

[bondhugula]

Suggestions for tweaking the information printed welcome. This change does provide more information than just "<block argument>" but still not enough (as we aren't identifying its Block in any way).

[bondhugula]

These build failures appear unrelated.

[bondhugula]

Trigger harbormaster build check.

[bondhugula]

Rebase.

mlir/lib/IR/AsmPrinter.cpp

 void IntegerSet::print(raw_ostream &os) const {
   ModulePrinter(os).printIntegerSet(*this);
 }
 
 void Value::print(raw_ostream &os) {
   if (auto *op = getDefiningOp())
     return op->print(os);
   // TODO: Improve this.
-  assert(isa<BlockArgument>());
-  os << "<block argument>\n";
+  BlockArgument arg = this->cast<BlockArgument>();
+  os << "<block argument> of type '" << arg.getType()
+     << "' at index: " << arg.getArgNumber() << '\n';
 }
 void Value::print(raw_ostream &os, AsmState &state) {
   if (auto *op = getDefiningOp())
     return op->print(os, state);
 
   // TODO: Improve this.
-  assert(isa<BlockArgument>());
-  os << "<block argument>\n";
+  BlockArgument arg = this->cast<BlockArgument>();
+  os << "<block argument> of type '" << arg.getType()
+     << "' at index: " << arg.getArgNumber() << '\n';
 }
 
 void Value::dump() {
   print(llvm::errs());
   llvm::errs() << "\n";
 }
 
 void Value::printAsOperand(raw_ostream &os, AsmState &state) {