[X86] Add back the assert from r320830 that was reverted in r320850

Hopefully r320864 has fixed the offending case that failed the assert.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320898 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed