llvm dev - Nov 2020 - [MachineLICM] how to add target-special constraints

Hi,
I am facing a problem that MachineLICM is hoisting some MachineInstrs that are
special for my target. I do not want to disable the optimization so I am looking
for some solutions to avoid such hoisting. I have not seen anyone create
subclass for MachineLICMBase either.
Looking at the implementation in MachineLICMBase::ProcessMI(), I could not find
a place to add target specific constraints (for example, defines to certain
registers cannot be hoisted). There is no target-specific callbacks I can
overwrite.
One possible fix I can think of is to initialize PhysRegClobbers with the
special registers. However, that will require a lot of changes to the common
codebase, for example, I might need to add some helper functions in
TargetRegisterInfo. I am not sure if that is a good idea.
Any suggestions are highly appreciated. 
Many thanks,PW
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