[RISCV] Use custom isel for vector indexed load/store intrinsics.

There are many legal combinations of index and data VTs supported
for these intrinsics. This results in a lot of isel patterns in
RISCVGenDAGISel.inc.

By adding a separate table similar to what we use for segment
load/stores, we can more efficiently manually select these
intrinsics. We should also be able to reuse this table scalable
vector gather/scatter.

This reduces the llc binary size by ~56K.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D97033