What to remove after tree-ssa is merged?

[Richard Kenner]

    The reasoning so far has been: If it can be expressed in RTL,
    then there has to be an equivalent representation in trees.
    So basically we hope that PLACEHOLDER_EXPRs can be expanded
    to an equivalent tree without PLACEHOLDER_EXPPs, i.e. making
    this node an Ada language-specific tree node.  

No, I don't think that's the best approach because of its critical
role inside expressions for offsets and sizes.