This is a somewhat form-fitted fix for jruby#9293 that only
assists with line numbers of the then and else bodies of IfNode. In
the linked issue, a trailing `if` or `unless` modifier will be used
as the line number that an associated call gets associated with,
causing stack output like from `caller_locations` to be incorrect.
In CRuby, this newline propagation appears to be done during the
`reduce_nodes` function, but that function has many side effects
in its macro-heavy logic and an initial attempt to port it did not
fix the issue in question. Instead, I just detect this specific
case and propagate the newline manually.

There may be other such cases, and once we encounter those there
may be a better justification for formally duplicating the newline
side-effect logic from `reduce_node`. For now, this quick fix is
probably good enough.

Fixes jruby#9293