useful to represent a variable that is const in the source but can't be constant
in the IR because of a non-trivial constructor. If globalopt evaluates the
constructor, and there was an invariant.start with no matching invariant.end
possible, it will mark the global constant afterwards.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150794 91177308-0d34-0410-b5e6-96231b3b80d8