version_40: metadce fixes (#1329)

* ignore missing imports (the wasm may have already had them optimized out)

 * handle segments that hold on to globals (root them, for now, as we can't remove segments)

 * run reorder-functions, as the optimal order may have changed after we dce

* fix global, global init, and segment offset reachability

* fix import rooting and processing - imports may be imported more than once