[X86] Make 128/256-bit extract_subvector Legal instead of Custom. Move...

[X86] Make 128/256-bit extract_subvector Legal instead of Custom. Move combining with BUILD_VECTOR from Legalization to DAG combine

EXTRACT_SUBVECTOR was marked Custom solely so we could combine it with BUILD_VECTOR operations to create smaller BUILD_VECTORS during Legalization. But that sort of combining should really be done by the DAG combiner.

This patch adds the last piece of needed supported DAG combine to handle this. Once that's done we can make the EXTRACT_SUBVECTOR operations Legal.

Differential Revision: https://reviews.llvm.org/D37197

llvm-svn: 311893