Avoid needless refcount and mortialization on pp_anonhash

pp_anonhash can be used to construct both bare hashes and hashrefs. In
the hashref case, it used to create an HV and mortalize it. Then it went
through the parameters on the stack and copied them into the hashref.
This can throw exceptions which would make the HV leak if it hadn't been
mortalized.

After potentially copying the arguments, pp_anonhash would then in the
hashREF case increment the refcount on the HV *again*, create an RV for
the HV, and mortalize that RV before pushing it on the stack.

Instead, we can get away with constructing the HV and only mortalizing
it if there's no mortalizable ref to clean up if there's an exception.
This should remove a fair fraction of work in the common case of empty {}
hash ref construction.