It turns out that when using the constant quadword format, we sometimes
need to swap the first and second constants in the quadword, or else
some of the bits are corrupted when loading either constant. Doing this
is tricky, since it has to happen before encoding the instructions
(because we need to know which slot to load constants from) and hence we
have to duplicate some of the logic for encoding immediates to figure
out ahead of time which constants get grouped together. We also have to
insert dummy constants earlier, since a dummy constant may have to be
swapped with a non-dummy one.