NewGVN: Don't try to kill off the stored value of stores when

processing the congruence class of the store.
Because we use the stored value of a store as the def, it isn't dead
just because it appears as a def when it comes from a store.

Note: I have not hit any cases with the memory code as it is where
this breaks anything, just because of what memory congruences we
actually allow.  In a followup that improves memory congruence,
this bug actually breaks real stuff (but the verifier catches it).

llvm-svn: 299300