Fix else transformation to use all old variables

[rtjoa]

Consider the following program:

x = 0
c = 1
while c == 1:
    if c == 1:
        x = x + 1
        c = Bernoulli(1/2)
    else:
        x = x + 1
    end
end

The else branch should be dead code. However, its inclusion causes the analysis to change from E(x) = 0; 2 - 2/2**n to the incorrect result E(x) = 0; 3 - 3/2**n. This happens because the else transformation does not replace c with the variable storing its old value in the transformed else condition !(c == 1).

This PR makes the appropriate fix and adds the above example as a regression test.

[mmsbrggr]

Hey Ryan. Thanks for your contribution. Great catch!