[perl #87708] $tied == $tied

[perl #87708] $tied == $tied

This is only part of #87708.

This fixes the + operator outside of any ‘use integer’ scope when the
same tied scalar is used for both operands and returns two different
values. Before this commit, get-magic would be called only once and
the same value used. In 5.12.x it just worked.

I tried modifying pp_eq throughout to take this case into account,
but it made the most common cases slightly slower, presumably because
of the extra checks. So this follows the same temp sv method that I
used for pp_add (in 4c3ac4b and 837c879), which, though slowing down
this edge cases due to the extra allocation, leaves the most common
cases just as fast. (And, in case my benchmarks were unreliably [not
unlikely], this method is also safer, as it has less chance of getting
different code paths wrong.)