[SLP][RISCV] Account for offset folding in getPointersChainCost

[SLP][RISCV] Account for offset folding in getPointersChainCost

For a GEP in a pointer chain, if:
1) a pointer chain is unit-strided
2) the base pointer wasn't folded and is sitting in a register somewhere
3) the distance between the GEP and the base pointer is small enough and
   can be folded into the addressing mode of the using load/store

Then we can exclude that GEP from the total cost of the pointer chain,
as it will likely be folded away.

In order to check if 3) holds, we need to know the type of memory access
being made by the users of the pointer chain. For that, we need to pass
along a new argument to getPointersChainCost. (Using the source pointer
type of the GEP isn't accurate, see https://reviews.llvm.org/D149889 for
more details).

Also note that 2) is currently an assumption, and could be modelled more
accurately.

This prevents some unprofitable cases from being SLP vectorized on
RISC-V by making the scalar costs cheaper and closer to the actual
codegen.

For now the getPointersChainCost hook is duplicated for RISC-V to prevent
disturbing other targets, but could be merged back in and shared with
other targets in a following patch.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D149654