Consider ad hoc aliasing when building RegUnits.

Register units can be used to compute if two registers overlap:

  A overlaps B iff units(A) intersects units(B).

With this change, the above holds true even on targets that use ad hoc
aliasing (currently only ARM). This means that register units can be
used to implement regsOverlap() more efficiently, and the register
allocator can use the concept to model interference.

When there is no ad hoc aliasing, the register units correspond to the
maximal cliques in the register overlap graph. This is optimal, no other
register unit assignment can have fewer units.

With ad hoc aliasing, weird things are possible, and we don't try too
hard to compute the maximal cliques. The current approach is always
correct, and it works very well (probably optimally) as long as the ad
hoc aliasing doesn't have cliques larger than pairs. It seems unlikely
that any target would need more.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156763 91177308-0d34-0410-b5e6-96231b3b80d8

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