Constify some things in preparation for CodeGenSubRegIndex to be stored by value in their container, removing the indirection

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222949 91177308-0d34-0410-b5e6-96231b3b80d8
Use deque<T> rather than vector<T*> since it provides the same invalidation semantics (at least when removal is not needed) without the extra indirection/ownership complexity

Order matters for this container, it seems (using a forward_list and
replacing the original push_backs with emplace_fronts caused test
failures). I didn't look too deeply into why.

(& in retrospect, I might go back & change some of the forward_lists I
introduced to deques anyway - since most don't require removal, deque is
a more memory-friendly data structure (moderate locality while not
invalidating pointers))

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222950 91177308-0d34-0410-b5e6-96231b3b80d8