JIT: improve handling of stores with a known address

1. Genericize the immediate-address handling, and make it apply to float stores too. Optimize some of the FIFO stores, etc a bit.

2. If the offset of a store is 0x8000, the write is almost always a FIFO store; optimize this case (and set up a fallback path if we're wrong). (this has been dropped, because I can't get it to not break with NBA 2k11)