[EDSC] Fix Stmt::operator= and allow DimOp in For loops

This CL fixes 2 recent issues with EDSCs:
1. the type of the LHS in Stmt::operator=(Expr rhs) should be the same as the (asserted unique) return type;
2. symbols coming from DimOp should be admissible as lower / upper bounds in For

The relevant tests are added.

PiperOrigin-RevId: 234750249