Introduction of cost based decision on filesort vs index for UPDATE
statements changed detection of the fact that the index used to scan the
table is being updated. The new design missed the case of index merge
when there is no single index to check. That was worked until a recent
change in InnoDB after which it went into infinite recursion if update of
the used index wasn't properly detected.

The fix consists of 'used key being updated' detection code from 5.1.