SelectionDAG: Introduce PersistentID to SDNode for assert builds.

SelectionDAG: Introduce PersistentID to SDNode for assert builds.

This gives us more human readable numbers to identify nodes in debug
dumps.

Before:
  0x7fcbd9700160: ch = EntryToken

  0x7fcbd985c7c8: i64 = Register %RAX

   ...

      0x7fcbd9700160: <multiple use>
    0x7fcbd985c578: i64,ch = MOV64rm 0x7fcbd985c6a0, 0x7fcbd985cc68, 0x7fcbd985c200, 0x7fcbd985cd90, 0x7fcbd985ceb8, 0x7fcbd9700160<Mem:LD8[@foo]> [ORD=2]

  0x7fcbd985c8f0: ch,glue = CopyToReg 0x7fcbd9700160, 0x7fcbd985c7c8, 0x7fcbd985c578 [ORD=3]

    0x7fcbd985c7c8: <multiple use>
    0x7fcbd985c8f0: <multiple use>
    0x7fcbd985c8f0: <multiple use>
  0x7fcbd985ca18: ch = RETQ 0x7fcbd985c7c8, 0x7fcbd985c8f0, 0x7fcbd985c8f0:1 [ORD=3]

Now:
  t0: ch = EntryToken

  t5: i64 = Register %RAX

    ...

      t0: <multiple use>
    t3: i64,ch = MOV64rm t10, t12, t11, t13, t14, t0<Mem:LD8[@foo]> [ORD=2]

  t6: ch,glue = CopyToReg t0, t5, t3 [ORD=3]

    t5: <multiple use>
    t6: <multiple use>
    t6: <multiple use>
  t7: ch = RETQ t5, t6, t6:1 [ORD=3]

Differential Revision: http://reviews.llvm.org/D12564

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248010 91177308-0d34-0410-b5e6-96231b3b80d8