Refactor the code that checks that all operands of a node are UNDEFs.

Add a micro-optimization to getNode of CONCAT_VECTORS when both operands are undefs.
Can't find a testcase for this because VECTOR_SHUFFLE already handles undef operands, but Duncan suggested that we add this.

Together with Michael Kuperstein <michael.m.kuperstein@intel.com>

llvm-svn: 160229