[mlir][llvm] Add vector insert/extract intrinsics

These intrinsics will be needed to convert between fixed-length vectors
and scalable vectors.

This operation will be needed for VLS (vector-length specific)
vectorization, when interfacing with vector functions or intrinsics that
take scalable vectors as operands in a context where the length of our
vectors is known or assumed at compile time, but we still want to
generate scalable vector instructions.

Differential Revision: https://reviews.llvm.org/D127100