[LegalizeTypes] When promoting the result of EXTRACT_SUBVECTOR, also check if...

[LegalizeTypes] When promoting the result of EXTRACT_SUBVECTOR, also check if the input needs to be promoted. Use that to determine the element type to extract.

Otherwise we end up creating extract_vector_elts that then each need to have their input promoted. This can lead to truncates needing to be emitted for each of those.

But we already emitted any_extends when we legalized the extract_subvector. So now we have pairs of any_extend+trunc that partially cancel. But depending on how DAGCombiner visits them we can get weird results.

By promoting the input at the same time we can create only a single any_extend or truncate.

There's one regression in the vector-narrow-binop.ll case, but that looks easy to fix with a follow up patch.

llvm-svn: 354647