[llvm] [RISCV][GISel] Instruction select for vector G_ADD, G_SUB (PR #74114)

[Jiahan Xie via llvm-commits]

jiahanxie353 wrote:

okay, there's nothing showing up under "Reviewers" to request your review, so I'll just @michaelmaitland @topperc 

https://github.com/llvm/llvm-project/pull/74114