Maybe a refcount leak

Summary: While testing objprof I noticed that interp mode gives me the right refcount while jit mode doesn't.
Specifically, for string keys of array-data, they have one too many refcounts.
I tried to triage this with debug prints and I reached addElemStringKeyHelper (an op references from hhbc).
It looks half "copied-pasted" from the addElemIntKeyHelper except it doesn't dec the key's ref count.

I added key->decRefCount() and tested that home.php still works on my sandbox and that it fixed the refcount issue for me.
Now I'm diffing this and also running a perflab.

Image- https://www.latest.facebook.com/pxlcld/lDgL

Reviewed By: @markw65

Differential Revision: D1573712