[Bug tree-optimization/77485] Missed dead store elimination of aggregate store followed by partial stores

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77485

--- Comment #4 from Jeffrey A. Law <law at redhat dot com> ---
So my patches for 33562 will detect the partial dead store in "foo", but I
never wrote the bits to narrow partial dead stores.

The difficulty in optimizing this particular case will be rewriting the
CONSTRUCTOR node.  Though it may be as simple as walking down the CONSTRUCTOR
and splicing out components which correspond to dead assignments -- I'll have
to familiarize myself with the guts of how to walk CONSTRUCTOR nodes. 
Hopefully they're at least in-order and extracting byte offsets is easy :-)