invoke.texi: Document -fdelete-null-pointer-checks

invoke.texi: Document -fdelete-null-pointer-checks

        * invoke.texi: Document -fdelete-null-pointer-checks
        * toplev.c (flag_delete_null_pointer_checks): New.
        (f_options): Add entry for -fdelete-null-pointer-checks.
        (rest_of_compilation): Conditionalize null pointer check
        elimination on flag_delete_null_pointer_checks.
        (main): If -O2 or greater, enable -fdelete-null-pointer-checks

From-SVN: r29631