[mlir][vector] Disable vector.flat_transpose for scalable vectors (#115338)
Disables `vector.flat_transpose` for scalable vectors. As per the docs:
> This is the counterpart of llvm.matrix.transpose in MLIR
I'm not aware of any use of any matrix-multiply intrinsics in the
context of scalable vectors, hence disabling.
Note, this is a follow-on for #102573 in which I disabled
`vector.matrix_multiply`.