[mlir] Add value_begin/value_end methods to DenseElementsAttr

Currently DenseElementsAttr only exposes the ability to get the full range of values for a given type T, but there are many situations where we just want the beginning/end iterator. This revision adds proper value_begin/value_end methods for all of the supported T types, and also cleans up a bit of the interface.

Differential Revision: https://reviews.llvm.org/D104173