[NewGVN] Fixup store count for the `initial` congruency class.

It was always zero. When we move a store from `initial` to its
own congruency class, we end up with a negative store count, which
is obviously wrong.
Also, while here, change StoreCount to be signed so that the assertions
actually fire.

Ack'ed by Daniel Berlin.

llvm-svn: 291725