[ValueTracking] Turn a test into an assertion.

As discussed with Chad, this should never happen, but this
assertion is basically free, so, keep it around just in case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310493 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed