Consider using llvm.vector.insert and llvm.vector.extract in slice_vector.

[zvookin]

In the SVE2 branch, slice_vector uses newish llvm.vector.insert and llvm.vector.extract intrinsics for the scalable vector case. We should evaluate whether this is a better approach to use always.

[steven-johnson]

Is this still relevant, given that the SVE2 branch has landed?