Fix for Mantis #25606.

pexpr.pas, postfixoperators:
  * try_type_helper: also allow type helpers for block type bt_const

+ added test


git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@29176 3ad0048d-3df7-0310-abae-a5850022a9f2