The problem was that bits of the destructive equality propagation
optimization weren't being undone after the execution of a stored
program. Modifications to the parse tree that are based on transient
properties must be undone to enable the re-execution of stored
programs.

The solution is to cleanup any references to predicates generated
by the equality propagation during the execution of a stored program.

mysql-test/r/trigger.result:
  Add test case result for Bug#51650.
mysql-test/t/trigger.test:
  Add test case for Bug#51650.
sql/item.cc:
  Remove reference to a equality predicate.