VMCore: add assert for miscompile

VMCore: add assert for miscompile

See PR11652. Trying to add this assert to
setSubclassData() itself actually prevented
the miscompile entirely, so it has to be here.

This makes the source of the bug more obvious
than the other asserts triggering later on did.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147390 91177308-0d34-0410-b5e6-96231b3b80d8