SelectionDAG: Handle out-of-bounds index in extract vector element

Summary: This patch correctly handles undef case of EXTRACT_VECTOR_ELT node where the element index is constant and not less than vector size.

Test Plan:
CodeGen for X86 test included.
Also one incorrect regression test fixed.

Reviewers: qcolombet, chandlerc, hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, llvm-commits

Differential Revision: http://reviews.llvm.org/D9250

llvm-svn: 236584