InstructionSimplify: Simplify a shuffle with a undef mask to undef

Summary:
Following the discussion in pr32486, adding the simplification:
 shuffle %x, %y, undef -> undef

Reviewers: spatel, RKSimon, andreadb, davide

Reviewed By: spatel

Subscribers: jroelofs, davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D32293

llvm-svn: 301764