MLBEDSW-5286 - MLCE: IndexError for ADD + TANH network

Resolves a bug where an IndexError would occur
if the same tensor was assigned to both IFM
and IFM2 of a binary elementwise operator
due to duplicates being allowed in operator
inputs but not in pass inputs.

Signed-off-by: Dwight Lidman <dwight.lidman@arm.com>
Change-Id: I39a6206a6252f6a848be9f9d4c5a8dc749c71699