[SLP]Introduce gather cost estimation function.

Introduced BoUpSLP::ShuffleCostEstimator::gather function as an initial
implementation of the gather/buildvector cost estimation for buildvector
nodes. It will allow to use general codegen infrastructure for better
cost estimation + it improves the cost estimation for the
gathers/buildvectors.

Improved part of D110978.

Differential Revision: https://reviews.llvm.org/D148174