Use RegUnits to compute overlapping registers.

Use RegUnits to compute overlapping registers.

TableGen already computes register units as the basic unit of
interference. We can use that to compute the set of overlapping
registers.

This means that we can easily compute overlap sets for one register at a
time. There is no benefit to computing all registers at once.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156960 91177308-0d34-0410-b5e6-96231b3b80d8