Move IF_STMT, CLEANUP_STMT out of C front end

[Richard Kenner]

    c-gimplify.c is getting smaller and smaller...

Speaking of which, once I finish my current round of changes (in the final
final testing process), I'd like to add DECL_STMT to GENERIC as DECL_EXPR:
there's a significant amount of processing for variable-sizes types and vars
that all front-ends have to do and it might as well be in common code.

Any thoughts?