Reduces false positives.

As an example:

result = "a" if TAINT else "c"

In AST, the assignment value is `IfExp(test=TAINT, body="a", orelse="c")`.

Even though `TAINT` is inside the assignment of `result`, it can't
actually taint `result` as it is part of the boolean test expression.

Previously, `result` would have been tainted, which was a false
positive.

We don't want to completely ignore the test though in case it contains a
sink function.

Therefore, if the test contains expressions we transform it as so:

result = "a" if b(c) + 2 else "d"

to the multi line:

__if_exp_0 = b(c) + 2
result = "a" if __if_exp_0 else "d"

This way if `b` is a sink and `c` is tainted we see a vulnerability, but
even if `c` is tainted we don't taint `result`.