Separate the codes into a table of distinct values
and an index into that table.
Pull the SSE calculation of the RDO loop and avoid
repeating for the same alpha values.

Change-Id: I8c4bd7eab6f8000e6aca9687d9190abc3e270c37
Signed-off-by: David Michael Barr <b@rr-dav.id.au>