[mlir][Transforms] Dialect conversion: Fix -debug crash (#125660)

Fix a crash in `ConversionPatternRewriter::replaceUsesOfBlockArgument`
when running with `-debug`. The block that owns the block argument can
be a detached block. In that case, do not attempt to print the name of
the owner op.

Author: matthias-springer

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -1708,10 +1708,13 @@ FailureOr<Block *> ConversionPatternRewriter::convertRegionTypes(
 void ConversionPatternRewriter::replaceUsesOfBlockArgument(BlockArgument from,
                                                            Value to) {
   LLVM_DEBUG({
-    Operation *parentOp = from.getOwner()->getParentOp();
-    impl->logger.startLine() << "** Replace Argument : '" << from
-                             << "'(in region of '" << parentOp->getName()
-                             << "'(" << from.getOwner()->getParentOp() << ")\n";
+    impl->logger.startLine() << "** Replace Argument : '" << from << "'";
+    if (Operation *parentOp = from.getOwner()->getParentOp()) {
+      impl->logger.getOStream() << " (in region of '" << parentOp->getName()
+                                << "' (" << parentOp << ")\n";
+    } else {
+      impl->logger.getOStream() << " (unlinked block)\n";
+    }
   });
   impl->appendRewrite<ReplaceBlockArgRewrite>(from.getOwner(), from,
                                               impl->currentTypeConverter);