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

[Michael Maitland via llvm-commits]

https://github.com/michaelmaitland approved this pull request.

LGTM with nit comment that `RISCV` -> `RISC-V` in commit description.

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