[SLP] Outline GEP chain cost modeling into new TTI interface - NFCI.

Cost modeling for GEPs should actually be target dependent but is currently
done inside SLP target-independent way.
Sinking it into TTI enables target dependent implementation.
This patch adds new TTI interface and implementation of the basic functionality
trying to retain existing cost modeling.

Differential Revision: https://reviews.llvm.org/D144770