[RISCV] Initial support for insert/extract subvector

This patch handles cast-like insert_subvector & extract_subvector
in which case:
1. index starts from 0.
2. inserting a fixed-width vector into a scalable vector,
   or extracting a fixed-width vector from a scalable vector.

Reviewed By: craig.topper, frasercrmck

Differential Revision: https://reviews.llvm.org/D96352