MLBEDSW-10106: Improve HLCS featuremap mapping mechanism.

HLCS generation uses a positional equivalence between
tensor index and tensor usage to map ifms from Schedule
IR to HLCS. This doesn't scale and required looping over
a fixed number of IFMs.

 - Switch from positional to tensor-usage based IFM
   mappings.
 - Allow potentially unlimited input IFMs for HLCS
   operators.
 - Improved field layout of HLCFeaturemap to reduce
   memory wasted by padding.

Signed-off-by: Philip Hall <philip.hall@arm.com>
Change-Id: Iea517eb8994e70ba3d8c0618dfd487706e785b60