Unroll AKExists into a series of comparisons on small static arrays

Summary: This is meant to go hand-in-hand with my other change to transform calls
to `in_array` into AKExists, though of course this will benefit organic
uses of AKExists too.

The CFG this generates is suboptimal. It generates a series of diamonds
(one per element of the haystack array) whereas it would ideally result
in something like this:

```
B1 --F--> B2 --F--> B3 --F--+
|         |         |       |
T         T         T       |
|         |         |       |
v         v         v       v
             Bdone
```

I'd have to expose additional API in IRBuilder to do this (since the
HhbcTranslator code would have to create and append its own Blocks) and
I wasn't sure it would be worthwhile. The worst that will happen is a
few redundant unconditional jumps, which we should actually be able to
optimize away downstream.

Reviewed By: @edwinsmith

Differential Revision: D1599893
Added support for ReflectionFunction::getClosureThis() [closes #3476]

Summary: PHP supports ReflectionFunction::getClosureThis() since version 5.4.0.
Added one test to slow section in norepo mode, since closure binding does not work in repo yet (3e290683d8504c74570343aee66da45863db8adf).
Closes #3476.
Closes https://github.com/facebook/hhvm/pull/3943

Reviewed By: @paulbiss

Differential Revision: D1608061