Fix order of if conditions to avoid unreachable code
If else if (preprocess_only)
is true, the more strict condition
else if (!include_file && preprocess_only)
won't be reached if it gets
checked after the shorter condition.
Exchange the two sections so that both code paths can be reached.
Fixes coverity CID 312499: Control flow issues (DEADCODE)
This was probably introduced in 7dcf013b / !743 (merged) which means we'll need to backport this fix to 3.0 and 3.1.