dillon1000/react

Fix for invalid mutable range in phi with backedge

Fixes the repro added in 947832009997bf9149e88e583c46cc39f6a6136c - previously 
when computing mutable ranges of phis, we didn't check that all operands had 
been visited. This meant that a backedge could allow a phi's mutable range to 
start at 0. Then in PropagateScopeDeps, we might see reject dependencies of a 
scope since they appeared to start after a scope — only because the scope's 
start was incorrectly too early. 

The fix here is to initially set phi.id mutable ranges based on only on operands 
that are already visited. Then, during/after the fixpoint iteration of 
InferMutableRanges, we start account for all operands since we know they've been 
visited at least once and have a real range.