[RISCV] Call SelectBaseAddr on the base pointer in the custom isel for vector loads and stores.

This will allow FrameIndex as the base address instead of
emitting a separate ADDI from isel. eliminateFrameIndex will likely turn
it back into an ADDI, but this makes things consistent with the
SDPatterns and VLPatterns.

I only tested one case for simplicity. I can test more if reviewers
want.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D97221