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

[Craig Topper via llvm-commits]

topperc wrote:

For scalable vector input and scalable vector output, G_EXTRACT_SUBVECTOR should produce a vector with a smaller minimum number of elements than the input vector. The element types are the same, but the total vector size is smaller.

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