[PowerPC] Update comment re: VSX copy-instruction selection

I've done some experimentation with this, and it looks like using the
lower-latency (but lower throughput) copy instruction is essentially always the
right thing to do.

My assumption is that, in order to be relatively sure that the higher-latency
copy will increase throughput, we'd want to have it unlikely to be in-flight
with its use. On the P7, the global completion table (GCT) can hold a maximum
of 120 instructions, shared among all active threads (up to 4), giving 30
instructions per thread.  So specifically, I'd require at least that many
instructions between the copy and the use before the high-latency variant is
used.

Trying this, however, over the entire test suite resulted in zero cases where
the high-latency form would be preferable. This may be a consequence of the
fact that the scheduler views copies as free, and so they tend to end up close
to their uses. For this experiment I created a function:

  unsigned chooseVSXCopy(MachineBasicBlock &MBB,
                         MachineBasicBlock::iterator I,
                         unsigned DestReg, unsigned SrcReg,
                         unsigned StartDist = 1,
                         unsigned Depth = 3) const;

with an implementation like:

  if (!Depth)
    return PPC::XXLOR;

  const unsigned MaxDist = 30;
  unsigned Dist = StartDist;
  for (auto J = I, JE = MBB.end(); J != JE && Dist <= MaxDist; ++J) {
    if (J->isTransient() && !J->isCopy())
      continue;

    if (J->isCall() || J->isReturn() || J->readsRegister(DestReg, TRI))
      return PPC::XXLOR;

    ++Dist;
  }

  // We've exceeded the required distance for the high-latency form, use it.
  if (Dist > MaxDist)
    return PPC::XVCPSGNDP;

  // If this is only an exit block, use the low-latency form.
  if (MBB.succ_empty())
    return PPC::XXLOR;

  // We've reached the end of the block, check the successor blocks (up to some
  // depth), and use the high-latency form if that is okay with all successors.
  for (auto J = MBB.succ_begin(), JE = MBB.succ_end(); J != JE; ++J) {
    if (chooseVSXCopy(**J, (*J)->begin(), DestReg, SrcReg,
                      Dist, --Depth) == PPC::XXLOR)
      return PPC::XXLOR;
  }

  // All of our successor blocks seem okay with the high-latency variant, so
  // we'll use it.
  return PPC::XVCPSGNDP;

and then changed the copy opcode selection from:
    Opc = PPC::XXLOR;
to:
    Opc = chooseVSXCopy(MBB, std::next(I), DestReg, SrcReg);

In conclusion, I'm removing the FIXME from the comment, because I believe that
there is, at least absent other examples, nothing to fix.