[InstCombine] Add fold for extracting known elements from a stepvector

This patch allows folding stepvector + extract to the lane when the lane is
lower than the minimum size of the scalable vector. This fold is possible
because lane X of a stepvector is also X!
For instance, extracting element 3 of a <vscale x 4 x i64>stepvector is 3.

Differential Revision: https://reviews.llvm.org/D103153