bug#16544: Optimazation for is_mb_middle

[Jim Meyering]

Thank you for the updated patch.
I am attaching it (with a detailed commit/ChangeLog I have added,
and a NEWS entry), along with four additional commits.
One fixes an actual bug.  I expect to merge those commits into yours,
discarding their individual commit logs.  I leave them separate here
solely to make the changes easier to review.

By the way, I measured the speed-up before and after your change
on an idle AMD FX-4100 (best of 10), running these commands:

  yes $(printf '%078dm' 0)|head -1000000 > in
  for i in $(seq 10); do env LC_ALL=ja_JP.eucJP time src/grep -v m in; done

Before: 2.46
After: 0.36

Let me know if you see anything that can be improved,

Jim

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