fix gimplification of compound lvalues

[Richard Kenner]

    This is enough to complete a stage2 build of the Ada front end.  At
    which point we hit

    during stage3 build, which no doubt indicates miscompilation somewhere.
    I'd prefer someone else track down the miscompilation.  I'll fix it if
    it turns out to be middle-end related.

I just did a good Ada bootstrap a bit ago, but that was with a tree about
five days old and on x86-64.  I have to resolve the confict in my tree
with your change to tree-ssa-operands.c and then I'll try again with the
current tree and see how far I get.

The Ada tools don't build because of a change in the Ada part of the
front end that involves variable-sized returns.  I need to convert a
function call into a procedure call and that's going to be a little work.