The pretty-printing had swapped dst and src2; this was introduced when
we moved to RA2 (sorry about that! IMHO we should do something to
automate the mapping between regalloc arg collection and pretty
printing/emission).
`src2` comes at the end because it has a variable number of register
mentions; this is in line with how many of the other inst formats work.
Actual emitted code was never incorrect, just the pretty-printing.
Updated test golden outputs look correct to me now, including the one
that we saw was incorrect in #3945.
019ebf47b1