Add a RegisterTuples class to Target.td and TableGen.

A RegisterTuples instance is used to synthesize super-registers by
zipping together lists of sub-registers. This is useful for generating
pseudo-registers representing register sequence constraints like 'two
consecutive GPRs', or 'an even-odd pair of floating point registers'.

The RegisterTuples def can be used in register set operations when
building register classes. That is the only way of accessing the
synthesized super-registers.

For example, the ARM QQ register class of pseudo-registers could have
been formed like this:

  // Form pairs Q0_Q1, Q2_Q3, ...
  def QQPairs : RegisterTuples<[qsub_0, qsub_1],
                               [(decimate QPR, 2),
                                (decimate (shl QPR, 1), 2)]>;

  def QQ : RegisterClass<..., (add QQPairs)>;

Similarly, pseudo-registers representing '3 consecutive D-regs with
wraparound' look like:

  // Form D0_D1_D2, D1_D2_D3, ..., D30_D31_D0, D31_D0_D1.
  def DSeqTriples : RegisterTuples<[dsub_0, dsub_1, dsub_2],
                                   [(rotl DPR, 0),
                                    (rotl DPR, 1),
                                    (rotl DPR, 2)]>;

TableGen automatically computes aliasing information for the synthesized
registers.

Register tuples are still somewhat experimental. We still need to see
how they interact with MC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133407 91177308-0d34-0410-b5e6-96231b3b80d8

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