bug fixes for issues 30018 and 30822.

bug fixes for issues 30018 and 30822.

includes bugfixes pointed out during review:

 * Only `call_lifetime_start` for an alloca if the function entry does
   not itself initialize it to "dropped."

 * Remove `schedule_lifetime_end` after writing an *element* into a
   borrowed slice. (As explained by [dotdash][irc], "the lifetime end
   that is being removed was for an element in the slice, which is not
   an alloca of its own and has no lifetime start of its own")

[irc]: https://botbot.me/mozilla/rust-internals/2016-01-13/?msg=57844504&page=3