TargetTransformInfo: address calculation parameter for gather/scather

Address calculation for gather/scather in vectorized code can incur a
significant cost making vectorization unbeneficial. Add infrastructure to add
cost.
Tests and cost model for targets will be in follow-up commits.

radar://14351991
ARM cost model: Add cost for gather/scather

Fixes a 35% degradation compared to unvectorized code in
MiBench/automotive-susan and an equally serious regression on a private
image processing benchmark.

radar://14351991
X86 cost model: Add cost for vectorized gather/scather

radar://14351991