Remove unused while

This issue is related to #5, since it requires evaluation of expressions to know if the condition is false or not.

The rule also has to check for side effects. In case there are side effects in the condition and the result would be known, the condition could be set in a local variable to keep the side effects, and still remove the while statement.

print('foo')
while false do
    print('dead code')
end

Turns into:

print('foo')