DynamicMemRefType: iteration and access by indices

The methods to perform such operations have been implemented for the DynamicMemRefType in a way that is similar to the implementation for StridedMemRefType. Up until here one could pass an unranked memref to the library, and thus obtain a “dynamic” memref descriptor, but then there would have been no possibility to operate on its content.

Differential Revision: https://reviews.llvm.org/D131359