Commits on Jun 27, 2019

1. Use a more efficient iteration order for forward dataflow …

   Currently, dataflow begins by visiting each block in order of ID
   (`BasicBlock(0)`, `BasicBlock(1)`, etc.). This PR changes that initial
   iteration to reverse post-order. This ensures that the effects of all
   predecessors will be applied before a basic block is visited if the CFG
   has no back-edges, and should result in less total iterations even when
   back-edges exist. This should not change the results of dataflow
   analysis.
   
   The current ordering for basic blocks is pretty close to RPO
   already--`BasicBlock(0)` is already the start block, so the gains from
   this are pretty small, especially since we need to do an extra traversal
   up front.
   
   Note that some basic blocks are unreachable from the `START_BLOCK`
   during dataflow. We add these blocks to the work queue as well to
   preserve the original behavior.

   

   ecstatic-morse committed Jun 27, 2019
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