Adjust ScheduleDAGSDNodes::RegDefIter for patchpoints

PATCHPOINT is a strange pseudo-instruction. Depending on how it is used, and
whether or not the AnyReg calling convention is being used, it might or might
not define a value. However, its TableGen definition says that it defines one
value, and so when it doesn't, the code in ScheduleDAGSDNodes::RegDefIter
becomes confused and the code that uses the RegDefIter will try to get the
register class of the MVT::Other type associated with the PATCHPOINT's chain
result (under certain circumstances).

This will be covered by the PPC64 PatchPoint test cases once that support is
re-committed.

llvm-svn: 225907