Put union erasure in IlxGen phase

[dsyme]

This eliminates the "Union" rewrite phase and means IlxGen now directly generates IL.

This PR changes IL code generation and a bunch of baselines will need to be updated because the order in which locals are generated changes.

Next? There is still a very awkward, expensive and complexifying code transformation done as part of IlxGen from

"list of instructions" --> "structured code" --> "list of instructions" 

which I believe can be eliminated next.

[dsyme]

Note, 300 LOC disappear with this phase removal. There may also be a marginal performance improvement but I need to double check

[brodyberg]

I love it

On Tuesday, June 7, 2016, Don Syme notifications@github.com wrote:

Note, 300 LOC disappear with this phase removal

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[dsyme]

I am merging this since it is cleanup and all green