We want typedmemmove to use the heap bitmap to determine
where pointers are, instead of reinterpreting the type information.
The heap bitmap is simpler to access.

In general, typedmemmove will need to be able to look up the bits
for any word and find valid pointer information, so fill even after the
dead marker. Not filling after the dead marker was an optimization
I introduced only a few days ago, when reintroducing the dead marker
code. At the time I said it probably wouldn't last, and it didn't.

Change-Id: I6ba01bff17ddee1ff429f454abe29867ec60606e
Reviewed-on: https://go-review.googlesource.com/9885Reviewed-by: Austin Clements <austin@google.com>