Make the AsmPrinter print "<null type>" instead of crashing on null types

[mehdi_amini]

Summary

Even if this indicates in general a problem at call sites, the printer
is used for debugging and avoiding crashing is friendlier for example
when used in diagnostics or other printer.

[bondhugula]

mlir/lib/IR/AsmPrinter.cpp

1494

This change is very welcome, thanks! It would be good keep the message format consistent though - there are other places in the ASM printer that handle such null structures. Can we make the casing and angular brackets uniform? (changing to your format sounds good).

if (!value) {
  stream << "<<NULL>>";
  return;
}

if (!attr) {
  os << "<<NULL ATTRIBUTE>>";
  return;
}

void AffineExpr::print(raw_ostream &os) const {
  if (expr == nullptr) {
    os << "null affine expr";
    return;
  }

void AffineExpr::dump() const {
  print(llvm::errs());
  llvm::errs() << "\n";
}

void AffineMap::print(raw_ostream &os) const {
  if (map == nullptr) {
    os << "null affine map";
    return;
  }
  ModulePrinter(os).printAffineMap(*this);
}

[mehdi_amini]

Update format to <<NULL TYPE>> for consistency with attributes

mlir/lib/IR/AsmPrinter.cpp

 //===- AsmPrinter.cpp - MLIR Assembly Printer Implementation --------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
for (unsigned idx = 0, e = numElements; idx != e; ++idx) {
     printEltFn(attr, os, idx, isSigned);
     bumpCounter();
   }
   while (openBrackets-- > 0)
     os << ']';
 }
 
 void ModulePrinter::printType(Type type) {
+  if (!type) {
+    os << "<null type>";
+    return;
+  }
+
   // Check for an alias for this type.
   if (state) {
     StringRef alias = state->getAliasState().getTypeAlias(type);
     if (!alias.empty()) {
       os << '!' << alias;
       return;
     }
   }