Factorize condition insertion code out of build_check_stmt

Factorize condition insertion code out of build_check_stmt

This patch splits a new create_cond_insert_point_before_iter function
out of build_check_stmt, to be used by a later patch.

Tested by running cc1 -fasan on the test program below with and
without the patch and by inspecting the gimple output to see that
there is no change.

void
foo ()
{
  char foo[1] = {0};

  foo[0] = 1;
}

gcc/

	* asan.c (create_cond_insert_point_before_iter): Factorize out of ...
	(build_check_stmt): ... here.

From-SVN: r193439