[SLP]Fix/improve minbitwidth mapping to use TreeEntry as a key.

Currently, MinBWs map uses Value* as a key and stores mapping for each
value to be demoted. It make is it hard to get the actual MinBWs value
for the buildvector scalars(constants), since same constant might be
  used in different nodes with the different MinBWs values/decisions.
Also, it consumes extra memory for the vectorized values/instructions
 from the same nodes.
Better to map actual nodes. It fixes the bitwidth data fetching for
buildvector scalars and improves memory consumption/analysis time for
other instructions.