Style/IfUnlessModifier bad length calculation

[marcandre]

Unless I'm mistaken, Style/IfUnlessModifier doesn't take into account the comment in the length, to check if the modifier form will fit or not.

if foo # this comment's length is not taken into account
  bar
end
# corrected to
bar if foo # this comment's length is not taken into account

Ideally the code to calculate this should use the exact same method that actually computes the replacement...

See also #8282 and #8281