[CostModel] Sink intrinsic costs to base TTI.

Recommitting part of "[CostModel] Unify Intrinsic Costs."
de71def3

Move the switch statement from TTImpl::getIntrinsicCost to
TTI::getIntrinsicInstrCost. This enables BasicTTI to understand
more 'free' intrinsics instead of defaulting to a cost of 1.

Differential Revision: https://reviews.llvm.org/D80012