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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> is there a generic function to extract n out of vscale x n x m?

https://llvm.org/doxygen/classllvm_1_1details_1_1FixedOrScalableQuantity.html#ac4ab9dd9440c55bee1aa4a1195cee759

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