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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

As I have written this patch, I have made no commitment to `Extract a vector of destination type from the source vector.` I'm content to revisit this once someone has a use case, unless you'd like to address it now.

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