op.c: Force shared hash key optimisation for existing COWs

op.c: Force shared hash key optimisation for existing COWs

If COW scalars are becoming more prevalent (they are), the hash key
optimisation will be less and less likely to kick in if it is skipped
for anything already SvIsCOW.

The assumption is that such a scalar is already a shared hash key sca-
lar.  That is not true for copy-on-write scalars made such by the new
mechanism that allows existing non-COW scalar to be upgraded to such.

The purpose of using shared hash keys scalars here is that the precom-
puted hash is already stored in the scalar (ok, it points to it indi-
rectly), speeding up hash lookup.

New COW scalars don’t have that and offer no speedup here.

So skip the optimisation only when the COW scalar is a shared hash
key scalar.

All of the above applies to methods as well.