[RISCV] Support fixed-length vector sign/zero extension

This patch adds support for the custom lowering sign- and zero-extension
of fixed-length vector types. It does so through custom nodes. Since the
source and destination types are (necessarily) of different sizes, it is
possible that the source type is legal whilst the larger destination
type isn't. In this case the legalization makes heavy use of
EXTRACT_SUBVECTOR.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D97194