Fix generation of incorrect SQL syntax (n1 AS n2 AS n3)

Fixes FlipPHPBug.adl problem. I think this is what is intended here, but it should be checked. (It is not entirely clear to me why the only the source may be aliased)
Merge pull request #6 from AmpersandTarski/fix-flip-php-bug

Fix generation of incorrect SQL syntax (n1 AS n2 AS n3)