compaction: LCS: avoid needless work post reshape completion
compaction: LCS: avoid needless work post reshape completion

That's done by picking the ideal level for reshape input, such
that LCS won't have to either promote or demote data, because
the output level is not the best candidate for having the
size of the output data.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla