pan/mdg: Analyze helper termination after scheduling
Similar to the fix in 6bf8e960 ("pan/bi: Do helper termination analysis on clauses")
Though apparently a "theoretical issue only", fixes artefacts in DarkPlaces with both D3D9 and GL renderers.
Fixes: 9a7f0e26 ("pan/mdg: Use the helper invo analyze passes")
Originally c8ffaa57, though I couldn't be bothered fixing authorship for this commit.