Update of allowed RESCALE unsigned types

When we added the signless behavior of RESCALE, the available
conversions was made too wide. These new ERROR_IF checks bring
back the original intent, restricting to a small set of allowed
unsigned conversions.

Signed-off-by: Eric Kunze <eric.kunze@arm.com>
Change-Id: I83d00d9b71c85bd0e8f58ac15ace665a0ba87e02