[x86] Rip out some broken test cases for avx512 i1 store support.

It isn't reasonable to test storing things using undef pointers --
storing through those is at best "good luck" and really should be
transformed to "unreachable". Random changes in the combiner can
randomly break these tests for no good reason. I'm following up on the
original commit regarding the right long-term strategy here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213810 91177308-0d34-0410-b5e6-96231b3b80d8