Fix bug with "list($this) = .."

Summary: In some cases, trying to assign to "$this" via list assignemnt can cause
HHVM's bytecode emitter to encounter errors. This diff fixes the bytecode
emitter errors and makes HHVM match PHP5's behavior.

Reviewed By: @elgenie

Differential Revision: D1667214

Signature: t1:1667214:1415391466:d4ca1852a2ddc3ac560949d049f27c8c55c40b83