Add an assertion for the value enumerator (bitcode writer) NFC

I have an LTO snapshot (for which I don't have sources) that can't
be read back by LLVM. It seems the writer emitted broken bitcode
and this assertions aims at catching such cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274819 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed