MLBEDSW-10679: Restore allocation of CPU-only tensors

Restore allocation of CPU-only tensors when the
--separate-io-regions flag is not enabled. This matches the old
behavior.

This partially reverts d4c6dbd7.

Signed-off-by: Johan Gunnarsson <johan.gunnarsson@arm.com>
Change-Id: I4d36ce2197f4386d47cdd96a1859f3be44f63db6