JIT: support immediate stores

This is a bit of a WIP given that it.... turned out to be a lot bigger than I expected it to be, and I still need to bench it, but I'd love comments.

Basically I noticed there were tons of examples of code like this:

mov eax, 0x8
bswap eax, eax
mov [rdx+rbx], eax

This was clearly quite silly, so I decided to add support for immediate stores, a'la:

mov [rdx+rbx], 0x08000000

How hard could this be? Well, after a journey into the backpatcher, I can say "much harder than I originally thought..."