[GC] Optimized the core version of Parrot_gc_mark_PMC_alive() further, this time avoiding the function call if the PMC is already alive.
[GC] Optimized the core version of Parrot_gc_mark_PMC_alive() further, this
time avoiding the function call if the PMC is already alive.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@42006 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot