Since the input CFG is required to not have critical edges, such
blockparams are useless. The incoming blockparams can simply be replaced
with the vreg from the (unique) predecessor.

It's better to let the client's lowering code handle this so regalloc2
doesn't need to.

It isn't needed when there is only one successor.

This extends the fuzzer and checker to verify that branches that produce
outputs are properly supported and no moves are inserted after a branch.

This also checks the one edge case where a branch instruction may have
both operands and blockparams: when there is only one successor and that
successor only has one predecessor.