[x86] remove duplicate undef tests; NFC

These are incomplete and were made redundant with the consolidation in:
https://reviews.llvm.org/rL324678


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324754 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed