Simplify address mask setting logic in AddressableBits

I wrote some complicated conditionals for how to handle a partially
specified AddressableBits object in https://reviews.llvm.org/D158041 ,
and how to reuse existing masks if they were set and we had an
unspecified value.

I don't think this logic is the right thing to start
with.  Simplify back to the most straightforward
setting, where only the bits that have been specified
are set in the Process.