[mlir] Fix try_value_begin_impl for DenseElementsAttr

The previous implementation would still crash if the element type was
not iterable. This patch changes SparseElementsAttr to properly
implement `try_value_begin_impl` according to ElementsAttr and changes
DenseElementsAttr to implement `tryGetValues` as the basis for querying
element values.

Depends on D132904

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D132958