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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> Seems like I don't need to implement all the ValueMapping for some reason

It worked because the test cases have `regBankSelected: true` and have `vrb` filled in for the vector instructions: 
```
%2:vrb(<vscale x 1 x s8>) = G_ADD %0, %1
   ^^^
```

If you run the full gisel pipeline on a llvm program or `-run-pass=instruction-select` on MIR that just went through the IR translator, regbankselection will fail.

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