MachineOperand::setIsDebug should ensure the register is /not/ a definition

This currently unused function appeared to be asserting in the wrong
direction - DebugValues are never definitions of registers, only uses.
Curiously we don't perform any of these checks for the more common (&
actually used) case of MachineOperand::CreateReg (or other Create
functions).

llvm-svn: 184065