[PATCH] ISelDAG: Spot chain loops involving MachineNodes

[Tim Northover]

Previously, the DAGISel function WalkChainUsers was spotting that it had entered already-selected territory by whether a node was a MachineNode (amongst other things). Since it's fairly common practice to insert MachineNodes during ISelLowering, this was not the correct check.
    
Looking around, it seems that other nodes get their NodeId set to -1 upon selection, so this makes sure the same thing happens to all MachineNodes and uses that characteristic to determine whether we should stop looking for a loop during selection.
    
This should fix PR15840.

http://llvm-reviews.chandlerc.com/D1732

Files:
  lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
  lib/Target/AArch64/AArch64ISelDAGToDAG.cpp
  lib/Target/ARM/ARMISelDAGToDAG.cpp
  lib/Target/Hexagon/HexagonISelDAGToDAG.cpp
  lib/Target/MSP430/MSP430ISelDAGToDAG.cpp
  lib/Target/Mips/MipsISelDAGToDAG.cpp
  lib/Target/NVPTX/NVPTXISelDAGToDAG.cpp
  lib/Target/PowerPC/PPCISelDAGToDAG.cpp
  lib/Target/R600/AMDGPUISelDAGToDAG.cpp
  lib/Target/Sparc/SparcISelDAGToDAG.cpp
  lib/Target/SystemZ/SystemZISelDAGToDAG.cpp
  lib/Target/X86/X86ISelDAGToDAG.cpp
  test/CodeGen/X86/2012-01-16-mfence-nosse-flags.ll
  test/CodeGen/X86/i486-fence-loop.ll
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