Handle EXEC_IOLENGTH in doloop_contained_procedure_code.

Handle EXEC_IOLENGTH in doloop_contained_procedure_code.

This rather obvious patch fixes an ICE on valid which came about
because I did not handle EXEC_IOLENGTH as start of an I/O statement
when checking for the DO loop variable.  This is an 11 regression.

gcc/fortran/ChangeLog:

	PR fortran/99345
	* frontend-passes.c (doloop_contained_procedure_code):
	Properly handle EXEC_IOLENGTH.

gcc/testsuite/ChangeLog:

	PR fortran/99345
	* gfortran.dg/do_check_16.f90: New test.
	* gfortran.dg/do_check_17.f90: New test.