[MachineOperand] Fix LiveDebugVariables code after isRenamable change.

Fix code in LiveDebugVariables that was changing def MachineOperands to
uses, which will hit an assert for dead operands after the change to add
the renamable bit to MachineOperands.  Avoid the assert by clearing the
dead bit before changing the operand to a use.

Fixes issue reported in out of tree target by Jesper Antonsson at Ericsson.

llvm-svn: 321571