Fix `move-to-alias-register!' so that when an alias is reused for a

Fix `move-to-alias-register!' so that when an alias is reused for a
register that has no other aliases, then that alias is marked as
needing to be saved into its home.  Improve
`reuse-pseudo-register-alias!' so that it doesn't generate a move
instruction from a register to itself.  Fix a bug which prevented the
clear-map instructions from being inserted when the next node had
already been generated.