JIT: improve handling of stores with a known address, guess FIFO writes

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).
We didn't support gatherPipe > 4GB before; I just added the check because I wasn't 100% sure it would be in the low 2GB.