Get rid of unnecessary uses of var_get_dict_index().

Get rid of unnecessary uses of var_get_dict_index().

Some code used the result of var_get_dict_index() on a pair of variables to
tell whether the variables are the same.  This is unnecessary: v1 == v2
works just as well.

Other code used the result of var_get_dict_index() as part of a hash value
computation.  This is reasonable, but it is even better just to hash the
pointer to the variable.