Clarify why hv_common() tries to clear placeholders before calling hsplit().

Which makes me realise that if we clear placeholders, we may be able to
avoid the need to split at all.

(In fact, as hv_common() only adds one key to the hash, under the current
definition of DO_HSPLIT() which only considers total number of keys,
clearing any placeholder is going to be enough to drop the total number of
keys, and so no longer trigger the split. But we'll leave the code making a
second check, to avoid a tight coupling with the internals of DO_HSPLIT().)
In Perl_hv_common(), call S_clear_placeholders() directly.

This saves one call to HvPLACEHOLDERS_get().