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

[Craig Topper via llvm-commits]

topperc wrote:

>From the doc.

> "The index must be a constant multiple of the source vector’s minimum vector length."

This is wrong it should be a constant multiple of the _**destination**_ vector's minimum vector length.

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