This changes the approach taken to tail-merge the blocks
to always create a new block instead of trying to reuse some block,
and generalizes it to support dealing not with just the `ret` in the future.
This effectively lifts the CallBr restriction, although this isn't really intentional.
That showed that we miscompile blocks with their addresses taken in at least two places,
so two fixes had to be appliedis the only non-NFC change here, i'm not sure if it's reasonable/feasible to temporarily retain it.
Other restrictions of the transform remain.