Fix fill3

Problem to solve

The following discussion from !239 (merged) should be addressed:

• @smichels started a discussion: (+4 comments)

  @camilstaps I run into an issue implementing this. Can you please explain what the offset after the label for e.g. add_instruction_w_label_offset_w should be? For !238 (merged) I use 2 everywhere, assuming that this is correct as we know that the first node has 2 arguments for fill3 instructions. The same should however hold for fill2.

  For fill2__d01 and the constructor UT3_000 Int Int Int, I have to use (ab_size<<3)+2 to make it work. Can you explain this and how I should fill in the offset argument?

Proposal

Fix fill3.

Documentation

None.