Fixed several bugs.

- Immutable variables (i.e., iteration variable in FOR nodes)
  now work when referenced from nested functions.
- IF nodes where both branches are value-less are now handled
  correctly.
- same names are preserved in the LLVM output for easier debugging.
- OR and AND are handled correctly (this was already done before,
  it is done on the parser-level and I am writing this here just for
  the record).
- Removed some stale debug code.

With this commit, test/good/queens.tig compiles completely.