[DAG] WidenVectorResult - add support for ISD::VECTOR_INTERLEAVE/ISD::VECTOR_DEINTERLEAVE nodes

[RKSimon]

https://llvm.godbolt.org/z/ocrbvPb9j

[RKSimon]

CC @lukel97

[topperc]

This isn't going to be pretty to support. I think we might have to use extract_elt and build_vector.

[lukel97]

This isn't going to be pretty to support. I think we might have to use extract_elt and build_vector.

I'm not sure if we can do that for scalable vectors. We could go through the stack and use a masked gather with an appropriate step_vector for each result.

[RKSimon]

I had wondered about creating initial implementations of TargetLowering::expandVECTOR_INTERLEAVE/expandVECTOR_DEINTERLEAVE that just perform scalarization, at least for fixed vector types.

[topperc]

This isn't going to be pretty to support. I think we might have to use extract_elt and build_vector.

I'm not sure if we can do that for scalable vectors. We could go through the stack and use a masked gather with an appropriate step_vector for each result.

That doesn't work if the target doesn't support masked.gather.