Bring variable into scope if bound on all branches

[nachivpn]

Currently, this program is rejected although it is semantically valid:

x() -> 
    if
      true -> X = 1;
      false -> X = 3
    end,
    X.    %%line 24

with error:

test.erl: error in parse transform 'etc': {"Type Error: 'X' not bound on line 24

[nachivpn]

needs testing..