compiler/ppc/subprim: Apply most of the same LENGTH/LIST changes as on SPARC.
* Second verse, not quite same as the first, because PPC doesn't
have branch-delay slots.
* Move the error-trap out-of-line, remove the second type test,
rotate and partially unroll the loop so that the nromal exit just
falls out the bottom but we still don't have to branch into the
* Justification and explanation is the same as for the SPARC
versions, not too many commits ago.