Make needless_bool less aggressive for chained `if`s

In a function with multiple `if .. { return _; }`s with a trailing `!_` at the end, combining the last `if`+`!_` doesn't
make the code less complex necessarily. It just moves the "problem" one up. Piping this through the entire `if` chain in
the function either requires a non-trivial refactor or makes the code potentially less readable. Both are not acceptable
for a complexity lint.

This adds a check to the lint that makes sure that there is no other `if` statement before the last collapsible `if`
statement.