Minor changes to the EDSC API NFC

This CL makes some minor changes to the declarative builder Helpers:
1. adds lb, ub, step methods to MemRefView to avoid always having to go through std::get + range;
2. drops MemRefView& from IndexedValue which was just creating ownership concerns. Instead, an IndexedValue only needs to keep track of the ValueHandle from which a MemRefView can be constructed on-demand if necessary.

PiperOrigin-RevId: 237861493