Besides the weight, we also want to store up to two root registers per
unit. Most units will have a single root, the leaf register they
represent. Units created for ad hoc aliasing get two roots: The two
aliasing registers.
The root registers can be used to compute the set of overlapping
registers.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156792 91177308-0d34-0410-b5e6-96231b3b80d8