[MIBundle] Turn MachineOperandIteratorBase into a forward iterator.

This patch turns MachineOperandIteratorBase into a regular forward
iterator, which can be used with iterator_range.

It also adds mi_bundle_ops and const_mi_bundle_ops that return iterator
ranges over all operands in a bundle and updates a use of the old
iterator.

Reviewers: evandro, t.p.northover, paquette, MatzeB, arsenm

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D70561