[mlir][EDSC] NFC - Move StructuredIndexed and IteratorType out of Linalg

Summary:
This NFC revision will allow those classes to be reused to allow
building structured vector operations.

Reviewers: aartbik, ftynse

Subscribers: arphaman, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74279