[RISCV] Fix crash a vector add has a 4x sext and zext operand.

We can narrow one of the extends and keep the other original by
using a vwaddu.wv or vwadd.wv.

We were previously forgetting to keep the original operand and
instead took the source of its extend. This resulted in a type
mismatch that later failed with an impossible physical register copy.

To fix this I've refactored some code to maintain information about
whether the source needs to be extended at all for longer so we could
use it in materialize.

Differential Revision: https://reviews.llvm.org/D137106