[CodeGen] Move rollback capabilities outside of the rematerializer (#184341)

The rematerializer implements support for rolling back
rematerializations by modifying MIs that should normally be deleted in
an attempt to make them "transparent" to other analyses. This involves:

1. setting their opcode to DBG_VALUE and
2. setting their read register operands to the sentinel register.

This approach has several drawbacks.

1. It forces the rematerializer to support tracking these "dead MIs"
(even if support is optional, these data-structures have to exist).
2. It is not actually clear whether this mechanism will interact well
with all other analyses. This is an issue since the intent of the
rematerializer is to be usable in as many contexts as possible.
3. In practice, it has shown itself to be relatively error-prone.

This commit removes rollback support from the rematerializer and moves
those capabilities to a rematerializer listener than can be instantiated
on-demand and implements the same functionality on top of standard
rematerializer operations. The rematerializer now actually deletes MIs
that are no longer useful after rematerializations, and has support for
re-creating them on-demand without requiring additional tracking on its
part.