Introduce TargetRegisterInfo::getOverlaps(Reg), returning a list of all

Introduce TargetRegisterInfo::getOverlaps(Reg), returning a list of all
registers that alias Reg, including itself. This is almost the same as the
existing getAliasSet() method, except for the inclusion of Reg.

The name matches the reflexive TRI::regsOverlap(x, y) relation.

It is very common to do stuff to a register and all its aliases:

  stuff(Reg)
  for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias)
    stuff(*Alias);

That can now be written as the simpler:

  for (const unsigned *Alias = TRI->getOverlaps(Reg); *Alias; ++Alias)
    stuff(*Alias);

This change requires a bit more constant space for the alias lists because Reg
is included and because the empty alias list cannot be shared any longer.

If the getAliasSet method is eventually removed, this space can be reclaimed by
sharing overlap lists. For instance, %rax and %eax have identical overlap sets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121800 91177308-0d34-0410-b5e6-96231b3b80d8