This commutes the extension (`arith.extsi`, `arith.extui`) over the
following vector ops: `vector.broadcast`, `vector.shape_cast`,
`vector.transpose`, `vector.flat_transpose`.
I focused on these as I saw them getting created by vector unroll
patterns. Maybe except `vector.flat_transpose`.
Reviewed By: antiagainst
Differential Revision: https://reviews.llvm.org/D149534