Keep analyses live in unrolling (#1929)

Add code to keep the def-use manger and the inst-to-block mapping up-to-date. This means we do not have to rebuild them later.

To make this work, we will have to have to find places to update the
def-use manager. Updating the def-use manager is not straight forward
because we are unrolling loops, and we have circular references.

This forces one pass to register all of the definitions. A second one
to analyze the uses. Also because there will be references to the new
instructions in the old code, we want to register the definitions of the
new instructions early, so we can update the uses of the older code as
we go along.

The inst-to-block mapping is not too difficult. It can be done as instructions are created.

Fixes #1928.