branch! produces wrong target block numbers?

[femtomc]

I'm trying to write a no-op transform:

function block_transform(ir)
    ir = copy(ir)
    n_ir = IR()
    for (i, bb) in enumerate(ir.blocks)
        new_bb = block!(n_ir, i)
        for arg in bb.args
            argument!(new_bb, arg)
        end
        for stmt in bb.stmts
            push!(new_bb, stmt)
        end
        if !isempty(bb.branches)
            for branch in bb.branches
                branch.condition isa Nothing ? branch!(new_bb, branch.block, branch.args...; unless = nothing) : branch!(new_bb, branch.block, branch.args...; unless = branch.condition)
            end
        end
    end
    return n_ir
end

but the result is something weird when I use branch!

--- IR (foo2) ---
1: (%1)
  br 2 (0.0)
2: (%2)
  %3 = Main.BlockManip.Normal(0, 1)
  %4 = Main.BlockManip.rand(%3)
  %5 = %4 > 0.0
  br 4 unless %5
  br 3
3:
  %6 = Main.BlockManip.Normal(0, 1)
  %7 = Main.BlockManip.rand(%6)
  %8 = %2 + %7
  br 2 (%8)
4:
  return %2

--- Transformed ---
1: (%1)
  br 5 (0.0)
2: (%2)
  %3 = Main.BlockManip.Normal(0, 1)
  %4 = Main.BlockManip.rand(%3)
  %5 = %4 > 0.0
  br 6 unless %5
  br 5
3:
  %6 = Main.BlockManip.Normal(0, 1)
  %7 = Main.BlockManip.rand(%6)
  %8 = %2 + %7
  br 2 (%8)
4:
  return %2
5:

i.e. the block numbers produced in the branch statement by branch! seem to be off and I'm not sure where/if I made a mistake.

Edit: I also know this is inefficient, but can I use Pipe here and work with blocks like this?

Edit2: I think I see the crux of the issue - branch.block is referring to a branch in the original IR, not the new IR.