[CostModel] Add basic implementation of getGatherScatterOpCost.

Add a basic implementation of getGatherScatterOpCost to BasicTTIImpl.

The implementation estimates the cost of scalarizing the loads/stores,
the cost of packing/extracting the individual lanes and the cost of
only selecting enabled lanes.

This more accurately reflects the current cost on targets like AArch64.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D91984