FromBuilder should remove trivial yield step between two scan steps

[julianhyde]

FromBuilder should remove trivial yield between two scans. For example, the query

from i in [1, 2] yield {i} join j in [3, 4]

has a yield after a scan that defines i and before a scan that defines j. The yield is trivial because it returns a record with all variables in scope (just i) and therefore it should simplify to

from i in [1, 2] yield {i} join j in [3, 4]

But it does not remove the yield. At the point where it checks whether the yield is trivial, FromBuilder has already (wrongly) added j to the scope and therefore does not think yield {i} is trivial.