[SLP] Unify GEP cost modeling for load, store and GEP nodes.

Make a separate routine for GEPs cost calculation and make
the approach uniform across load, store and GEP tree nodes.
Additional issue fixed is GEP cost savings were applied twice
for ScatterVectorize nodes (aka gather load) making them look
unrealistically profitable for vectorization.

Differential Revision: https://reviews.llvm.org/D140789