[llvm] [GISEL] Fix bug in G_EXTRACT_SUBVECTOR definition (PR #108848)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> But extracting from a scalable vector of pearls should return a scalable vector of pearls.

Maybe this should be enforced in the MV.

https://github.com/llvm/llvm-project/pull/108848