[RISCV] Add cost model for vector insert/extract element.

This patch adds cost model for vector insert/extract element instructions. In RVV, we could use vector scalar move instruction to insert or extract the first element, and use vslide to move it. But for mask vector or i64 vector in i32 target, we need special instructions to make it.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D133007