[SLP] Explicitly pass AccessTy to getGEPCost

Building on D149889, this patch updates SLP to pass the vector type as
the AccessTy to getGEPCost.
This should have the effect of GEPs being costed for more often instead
of being treated as foldable into the address mode and thus free, as
some architectures, notably RISC-V, do not have offset+reg addressing
modes for vector memory accesses.

Note that in SLP, GEPs are costed in two places: getPointersChainCost
and GetGEPCostDiff.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D153570