dillon1000/react

Fix PruneMaybeThrows to update phi operand predecessor ids

When PruneMaybeThrows removes maybe-throw terminals, it's possible that the 
block in question reassigned a value s.t. it appears as a later phi operand. 
That phi has to be rewritten to reflect the updated predecessor block. 

Here we track these rewrites (transitively) and rewrite phi operands 
accordingly.