avoid multiple FETCHes

avoid multiple FETCHes

The fix 2d961f6deff7 for RT #5475 included a mechanism for the early
calling of get magic on something like

    $tied[0];

so that even though the element is used in void context, we still call
FETCH. Some people seem to rely on this.

However, the call to mg_get() didn't distinguish between a tiedelem
member retrieved from a tied array/hash, and a tiedscalar element
retrieved from a plain array/hash. In the latter case, the S_GSKIP
protection mechanism doesn't apply and a simple $foo = $h{tiedelem}
generated two calls to FETCH.

Fix this by only calling mg_get() on the element if it came from a *tied*
array/hash.

A side-effect of this fix is that the following no longer calls FETCH:

    my @plain_array;
    tie $plain_array[0], ....; # element 0 is now a tied scalar
    $plain_array[0]; # void context:  no longer calls FETCH.

This required one test in op/tie.t to be fixed up, but in general I think
this is a reasonable compromise.