[dataflow] partial register handling

[Paolo Bonzini]

> The rules should be simple.  Bits of the dest reg survive only if one of 
> the following is true.
> 
> - there is a STRICT_LOW_PART (of a SUBREG)
> 
> - there is a ZERO_EXTRACT (not necessarily of a SUBREG!)
> 
> - the subreg is part of a multiword subreg

So, this is a prototype patch that I would like to go in.

Paolo
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