Don't use pointer-pointers for the register use lists.

Don't use pointer-pointers for the register use lists.

Use a more conventional doubly linked list where the Prev pointers form
a cycle. This means it is no longer necessary to adjust the Prev
pointers when reallocating the VRegInfo array.

The test changes are required because the register allocation hint is
using the use-list order to break ties.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161633 91177308-0d34-0410-b5e6-96231b3b80d8