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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Our documentation does not mention fixed-length vectors:
https://llvm.org/docs/GlobalISel/GenericOpcode.html#g-extract-subvector

"This operation supports extracting a fixed-width vector from a scalable vector, but not the other way around."
comes from the ISD:: version.

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