Avoid name lookup warning

/home/markus/gcc/gcc/tree.c: In function ‘void inchash::add_expr(const_tree, inchash::hash&, unsigned int)’:
/home/markus/gcc/gcc/tree.c:8013:11: warning: name lookup of ‘i’ changed
      for (i = TREE_OPERAND_LENGTH (t) - 1; i >= 0; --i)
           ^
/home/markus/gcc/gcc/tree.c:7773:7: warning:   matches this ‘i’ under ISO standard rules
   int i;
       ^
/home/markus/gcc/gcc/tree.c:7869:16: warning:   matches this ‘i’ under old rules
       for (int i = 0; i < TREE_VEC_LENGTH (t); ++i)
                ^

From-SVN: r246519