DCE eliminating valid statement for ACATS c34007p

[Richard Kenner]

     If part way through gimplification you mark a variable as addressable,
     then you invalidate the gimplification of preceeding statements.  At
     least for variables whose type satisfies is_gimple_reg_type.

Yeah, that's what I realized in working on this problem.  Is it worth adding
a flag to show that this is a gimple-generated variable so we can have
an assert that we're not trying to mark it as addressable?