- In some cases, flow control optimization implemented in sp::optimize
    removes hreturn instructions, causing SQL exception handlers to:
      * never return
      * execute wrong logic
  - This patch overrides default short cut optimization on hreturn instructions
    to avoid this problem.


mysql-test/r/sp-code.result:
  Added test case
mysql-test/t/sp-code.test:
  Added test case
sql/sp_head.cc:
  Override opt_mark to get correct execution paths without jump short cut 
  optimization.
sql/sp_head.h:
  Added override sp_instr_hreturn::opt_shortcut_jump so that jump short cuts aren't
  performed on hreturn instructions operating on handlers which are set to CONTINUE
  after interruption.