functions in queries

Using MAX()/MIN() on table with disabled indexes (by ALTER TABLE)
results in error 124 (wrong index) from storage engine.

The problem was that optimizer use disabled index to optimize
MAX()/MIN(). Normally it must skip disabled index and perform
table scan.

This patch skips disabled indexes for min/max optimization.


mysql-test/r/myisam.result:
  Test case for BUG#20357.
mysql-test/t/myisam.test:
  Test case for BUG#20357.
sql/opt_sum.cc:
  Skip disabled/ignored indexes for min/max optimization.