[MachineInstrBundle] Update the comment for PhysRegInfo::DeadDef.

I missed read the comment when I commited r267621 and thought the
comment did not need update. Matthias kindly proved me wrong.
Fixing that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267638 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed