[msan] Workaround for invalid origins in shufflevector.

Makes origin propagation ignore literal undef operands, and,
in general, any operand we don't have origin for.

https://code.google.com/p/memory-sanitizer/issues/detail?id=56


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210472 91177308-0d34-0410-b5e6-96231b3b80d8