IR: Allow MDSubrange to have 'count: -1'

IR: Allow MDSubrange to have 'count: -1'

It turns out that `count: -1` is a special value indicating an empty
array, such as `Values` in:

    struct T {
      unsigned Count;
      int Values[];
    };

Handle it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229769 91177308-0d34-0410-b5e6-96231b3b80d8