[llvm] [RISCV] Transform build_vector((binop X_i, C_i)..) to binop (build_ve… (PR #67358)

[Philip Reames via llvm-commits]

preames wrote:

> Would this be a useful target generic canonicalisation?

In principle yes, and that's actually where I started with this.  However, the more I though, the more the costing seemed target specific.  We could expose better costing information, but that ends up being a huge chunk of work for minimalish value.  

https://github.com/llvm/llvm-project/pull/67358