Emit register unit root tables.

Emit register unit root tables.

Each register unit has one or two root registers. The full set of
registers containing a given register unit can be computed as the union
of the root registers and their super-registers.

Provide an MCRegUnitRootIterator class to enumerate the roots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157753 91177308-0d34-0410-b5e6-96231b3b80d8