[SelectionDAG] Simplify extract_subvector( insert_subvector ( Vec, In, Idx ), Idx ) -> In

If we are extracting a subvector that has just been inserted then we should just use the original inserted subvector.

This has come up in certain several x86 shuffle lowering cases where we are crossing 128-bit lanes.

Differential Revision: https://reviews.llvm.org/D24254

llvm-svn: 280715