IVGCVSW-2043 Merger using ACL for innermost concat axis
IVGCVSW-2043 Merger using ACL for innermost concat axis

 * Fixed unsigned to signed conversion error that was
   breaking master

Change-Id: I08d79d9bf131eb46ea0495371160698ab59c1721

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn