The JIT optimizer doesn't know about `_BINARY_OP_INPLACE_ADD_UNICODE`'s store

[brandtbucher]

Bug report

Our abstract interpreter should have a case for _BINARY_OP_INPLACE_ADD_UNICODE, since this "special" instruction includes a store to a fast local. This can mean that the local being stored to is left with stale information (for instance, when analyzing a = ""; a += "spam" the optimizer will incorrectly assume that a still has the value "").

CC @Fidget-Spinner.