powerpc optimiser incorrectly failed to realise that,
almost uniquely, RLWMI does a read-modify-write of its destination,
and therefore:
1. peep.c must mark the instruction as "read, read, rar" in copyu
2. peep.c must not allow the instruction to act like a move in subprop (in the sense that it fully-replaces a register's contents).

the error affected only programs that used right shifts of vlongs.