btr_cur_search_to_nth_level(), row_sel(): Do not bother to yield to
waiting exclusive lock requests on the adaptive hash index latch.
When the btr_search_latch was split into an array of latches
in MySQL 5.7.8 as part of the Oracle Bug#20985298 fix, the "caching"
of the latch across storage engine API calls was removed. Thus,
X-lock requests should have a good chance of becoming served, and
starvation should not be possible.

btr_search_guess_on_hash(): Clean up a debug assertion.