AMDGPU: Make undef legal for v2i16/v2f16

This is apparently necessary to stop undef from being
turned into a build_vector of 0s.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332195 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed