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

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

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D97192