Compute secondary sub-registers.

The sub-registers explicitly listed in SubRegs in the .td files form a
tree. In a complicated register bank, it is possible to have
sub-register relationships across sub-trees. For example, the ARM NEON
double vector Q0_Q1 is a tree:

  Q0_Q1 = [Q0, Q1],  Q0 = [D0, D1], Q1 = [D2, D3]

But we also define the DPair register D1_D2 = [D1, D2] which is fully
contained in Q0_Q1.

This patch teaches TableGen to find such sub-register relationships, and
assign sub-register indices to them. In the example, TableGen will
create a dsub_1_dsub_2 sub-register index, and add D1_D2 as a
sub-register of Q0_Q1.

This will eventually enable the coalescer to handle copies of skewed
sub-registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156587 91177308-0d34-0410-b5e6-96231b3b80d8

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