[SelectionDAG] Teach getNode to simplify a couple easy cases of EXTRACT_SUBVECTOR

Summary:
This teaches getNode to simplify extracting from Undef. This is similar to what is done for EXTRACT_VECTOR_ELT. It also adds support for extracting from CONCAT_VECTOR when we can reuse one of the inputs to the concat. These seem like simple non-target specific optimizations.

For X86 we currently handle undef in extractSubvector, but not all EXTRACT_SUBVECTOR creations go through there.

Ultimately, my motivation here is to simplify extractSubvector and remove custom lowering for EXTRACT_SUBVECTOR since we don't do anything but handle undef and BUILD_VECTOR optimizations, but those should be DAG combines.

Reviewers: RKSimon, delena

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29000

llvm-svn: 292876