AMDGPU: Put un-initiaized enumerators together in an enum definition.

Summary:
  For any enumerator whose definition does not have an initializer,
the associated value is the value of the previous enumerator plus one.
In order to avoid the possibility that two unrelated enumerators to
have the same value, we should try to cluster the uninitialized enumerators
together.

Reviewers: arsenm

Differential Revision
  https://reviews.llvm.org/D141643