[VectorExt] Change `to_layout` `shared_memory_conversion` (#24377)

Change how `shared_memory_conversion` is represented on the `to_layout`
operation. So far, a unit attribute was used to simply turn shared
memory conversion on/off. Now, the `shared_memory_conversion` takes an
attribute, which allows us to attach for example the GPU promotion type
(e.g. `use_global_load_dma`) to the operation.

The motivation is to use this additional information in analyses such as
https://github.com/iree-org/iree/pull/24227 as part of
https://github.com/iree-org/iree/issues/23782.

The representation is `AnyAttr` to (a) avoid a dependency from
`VectorExt` on `IREEGPU` and (b) to allow other pipelines to use the
attribute differently.

The PR also adds some verification to the `LoweringConfigAttr` to avoid
having to repeatedly check that the list of promoted operands and
promotion types has the same length.

Assisted-by: Codex

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Signed-off-by: Lukas Sommer <lukas.sommer@amd.com>