dillon1000/react

Infer reactive control dependencies

Updates `InferReactivePlaces` to infer control dependencies. We build on the 
formal definition of control dependencies, which is that statement S2 is 
control-dependent on statement S1 if S1 is in the post-dominance-frontier of S2. 
Intuitively, if S1 decides whether S2 is reached or not, then S1 is a control 
dependency of S2. The post dominance frontier of a given statement S is the set 
of statements which may or may not reach S, and captures the intuitive notion. 

We take advantage of phis: phis are the point where a variable may have multiple 
values depending on the path we took. If a phi is not already known to be 
reactive from data dependencies we check for control dependencies. Specifically 
we look at each phi operand. We check if the block that the operand came from 
has any reactive control dependencies, and if so we mark the phi itself as 
reactive. 

The post-dominance-frontier (PDF) algorithm requires walking the post-dominator 
tree a bunch, so we cache the PDF of blocks so that we don't have to recalculate 
on subsequent iterations. 

In addition, `InferReactiveIdentifiers` now uses the _union_ of its own 
inference plus the new `InferReactivePlaces` output when deciding what 
identifiers are reactive. This ensures that control dependencies are recorded 
correctly, fixing the previous test cases. The next diff adds the remaining 
features to InferReactivePlaces so that it can fully replace 
InferReactiveIdentifiers.