[mlir] Fix bug in partial dialect conversion

The discussion on forum:
https://llvm.discourse.group/t/bug-in-partial-dialect-conversion/4115

The `applyPartialConversion` didn't handle the operations, that were
marked as illegal inside dynamic legality callback.
Instead of reporting error, if such operation was not converted to legal set,
the method just added it to `unconvertedSet` in the same way as unknown operations.

This patch fixes that and handle dynamically illegal operations as well.

The patch includes 2 fixes for existing passes:

* `tensor-bufferize` - explicitly mark `std.return` as legal.
* `convert-parallel-loops-to-gpu` - ugly fix with marking visited operations
  to avoid recursive legality checks.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D108505