[mlir][std] Fold dim(dynamic_tensor_from_elements, %cst)

The shape of the result of a dynamic_tensor_from_elements is defined via its
result type and operands. We already fold dim operations when they reference
one of the statically sized dimensions. Now, also fold dim on the dynamically
sized dimensions by picking the corresponding operand.

Differential Revision: https://reviews.llvm.org/D91616