[SimplifyCFG] use helper for combining metadata

[Timothy Arceri via llvm-commits]

Rather than redefining known ids everywhere use the existing helper.

The helper should probably be renamed but its already used elsewhere
in SimplifyCFG so I've left as is for now.


Separate from this patch (but related). I'm also after suggestions on 
how to handle this bug:
https://bugs.llvm.org//show_bug.cgi?id=32001

The problem is the combine just throws away any metadata it doesn't know 
about. I could probably skip the sink if there are unknow metadata ids 
for the instruction, but that won't stop the issue popping up elsewhere.

How would this kind of thing be handled normally in LLVM? Is there a way 
to hook in target cpu specific functions into the combine calls? If 
someone can point me to an example of how this is normally done in LLVM 
that would be great.

Thanks,
Tim
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