[Fortran] RFC / RFA patch for using build_predict_expr instead of builtin_expect / PR 58721

[Dominique Dhumieres]

With the two patches at http://gcc.gnu.org/ml/fortran/2014-03/msg00004.html
I have been able to bootstrap my working tree, but I have several thousands
ICEs of the kind

/opt/gcc/work/gcc/testsuite/gfortran.dg/char_pointer_func.f90: In function 'afoo.constprop':
/opt/gcc/work/gcc/testsuite/gfortran.dg/char_pointer_func.f90:41:0: internal compiler error: in expand_builtin_expect, at builtins.c:4583
     allocate (ac1(1))

due to 

  /* When guessing was done, the hints should be already stripped away.  */
  gcc_assert (!flag_guess_branch_prob
              || optimize == 0 || seen_error ());

TIA

Dominique