Transpiler cleanup 15.07

[afetisov]

• remove UB const DUMMY_SP, use safe Span::call_site() instead;
• simplify argument parsing chains it.next().unwrap().parse().unwrap() into a single pattern match;
• extract common logic for adding architecture-dependent item usage in simd.rs into a separate function;
• various minor cleanups.

[afetisov]

If I split it up, all the comments you made here will be scrapped :-/

[kkysen]

None of them are too in-depth fortunately; I can re-make them.