[mlir] Fix a use after free when loading dependent dialects

The way dependent dialects are implemented is by recursively calling
loadDialect in the constructor. This means we have to reload from the
dialect table because the constructor might have rehashed that table.

The steps for loading a dialect are
  1. Insert a nullptr into loadedDialects. This indicates the dialect is
     loading
  2. Call ctor(). This recursively loads dependent dialects
  3. Insert the new dialect into the table.

We had a conflict between steps 2 and 3 here. You have to be extremely
unlucky though as rehashing is rare and operator[] does no generation
checking on DenseMap. Changing that to an iterator would've uncovered
this issue immediately.