[X86] Don't let LowerEXTRACT_SUBVECTOR call getNode for EXTRACT_SUBVECTOR.

This results in the simplifications inside of getNode running while we're legalizing nodes popped off the worklist during the final DAG combine. This basically makes a DAG combine like operation occur during this legalize step, but we don't handle something quite the same way. I think we don't recursively added the removed nodes to the DAG combiner worklist.

llvm-svn: 294929