James Miller eee7f3c732 Add and use a break critical edges transform
This is a fairly standard transform that inserts blocks along critical
edges so code can be inserted along the edge without it affecting other
edges. The main difference is that it considers a Drop or Call
terminator that would require an `invoke` instruction in LLVM a critical
edge. This is because we can't actually insert code after an invoke, so
it ends up looking similar to a critical edge anyway.

The transform is run just before translation right now.
2016-03-30 12:59:57 +13:00
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