The bug was this scenario:
1. Join optimizer picks a range plan on index IDX1
   (This index doesn't match the ORDER BY clause, so sorting will be needed)
2. Index Condition Pushdown pushes a part of WHERE down. The pushed
   condition is removed from SQL_SELECT::cond
3. test_if_skip_sort_order() figures that it's better to use IDX2
   (as it will match ORDER BY ... LIMIT and so will execute faster)

3.1 It sees that there was a possible range access on IDX2. It tries to
   construct it by calling SQL_SELECT::test_quick_select(), but alas,
   SQL_SELECT::cond doesn't have all parts of WHERE anymore.
   So it uses full index scan which is slow.

(The execution works fine because there's code further in test_if_skip_sort_order()
which "Unpushes" the index condition and restores the original WHERE clause.
It was just the test_quick_select call that suffered).