dillon1000/react

Fix EliminateRedundantPhi for cascading eliminated phis

This was the actual bug. When EliminateRedundantPhis eliminates a phi, it has to 
rewrite downstream usages of the phi id to the single operand id. We were 
correctly doing that in all but one place. When we iterate _downstream phis_, we 
were looking up the operands against the rewrite table, but not updating the phi 
operands themselves to the rewritten value. 

This fixes the bug, and incidentally fixes a test that has been broken for a 
while and nagging at me.