* fixes in optimize_op_const:
  * Do the sign extension of the 'a' parameter internally in a local variable,
    so that it does not become visible outside the function.
  * Use a zero extended version of 'a' (instead of sign extension) for the
    parameter of ispowerof2().



git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@26571 3ad0048d-3df7-0310-abae-a5850022a9f2