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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

I am well known blind, but for scalable vectors the type should be the same? It is not a cast. For fixed vectors, I agree.

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