[mlir][vector] Disable `BreakDownVectorBitCast` for scalable vectors (#122725)
`BreakDownVectorBitCast` leverages
* `vector.extract_strided_slices` + `vector.insert_strided_slices`
As these Ops do not support extracting scalable sub-vectors (i.e.
extracting/inserting a fraction of a scalable dim), it's best to bail
out.