Optimise scavenge_large_srt_bitmap

Very large modules can sometimes contain very large SRT bitmaps (this
is a separate problem that I need to look into).  The large bitmaps
often contain a lot of zeros, so this patch skips over empty words in
the bitmap.

It makes a dramatic difference in the particular example that I saw,
where an old gen GC was taking 0.5s before this change and 0.07s after
it.

(cherry picked from commit cf8e669b)