dillon1000/react

Repro for missing dep with while/if using externally declared "index" variable

What happens here is that the phi node for `i` has its mutable range set to 
start at 0, because it has a back edge and we haven't initialized the mutable 
ranges of all its operands yet when we iterate the operands and set range.start 
= min(start of operand starts). 

Then the corresponding scope has its range set to start at 0 too. When 
PropagateScopeDeps runs it sees that `b` is from instruction 1, which is after 
the start of the scope (0), so it thinks `b` isn't a valid dependency. 

The fix is in InferMutableRanges, where we need to make sure that phis ignore 
their operand's ranges until those ranges are initialized.