fix code coverage bug in tail position and `else`

[JeffBezanson]

This was due to lowering keeping the same location info for the inserted return or goto statement, even though the last seen location might not have executed.

Also fixes inliner handling of the sentinel 0 value for code locations.

[JeffBezanson]

This seems to still be subtly wrong. Now I'm thinking the right way to fix this might be to put location nodes at the end of if blocks like we do for for and while.

[JeffBezanson]

Ok I fixed the fix, now this might be good enough on its own.