This is the first PR that introduces `ub.poison` vectors as part of a
rewrite/conversion pattern in the Vector dialect. It replaces the
`arith.constant dense<0>` vector initialization for
`vector.insert_slice` ops with a poison vector.
This PR depends on all the previous PRs that introduced support for
poison in Vector operations such as `vector.shuffle`, `vector.extract`,
`vector.insert`, including ODS, canonicalization and lowering support.
This PR may improve end-to-end compilation time through LLVM, depending
on the workloads.
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