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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

"This operation supports extracting a fixed-width vector from a scalable vector, but not the other way around."
Then the types differ.

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

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