InstructionSimplify: 'extractelement' with an undef index is undef

Summary:
An undef extract index can be arbitrarily chosen to be an
out-of-range index value, which would result in the instruction being undef.

This change closes a gap identified while working on lowering vector permute intrinsics
with variable index vectors to pure LLVM IR.

Reviewers: arsenm, spatel, majnemer

Reviewed By: arsenm, spatel

Subscribers: fhahn, nhaehnle, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D40231

llvm-svn: 319910