[RISCV] Unify scalable- and fixed-vector INSERT_SUBVECTOR lowering

This patch unifies the two disparate paths for lowering INSERT_SUBVECTOR
operations under one roof. Consequently, with this patch it is possible to
support any fixed-length subvector insertion, not just "cast-like" ones.

As before, support for the insertion of mask vectors will come in a
separate patch.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D97543