[InstCombine] After merging store into successor, queue prev. store to be visited (PR46661)

We can happen to have a situation with many stores eligible for transform,
but due to our visitation order (top to bottom), when we have processed
the first eligible instruction, we would not try to reprocess the previous
instructions that are now also eligible.

So after we've successfully merged a store that was second-to-last instruction
into successor, if the now-second-to-last instruction is also a such store
that is eligible, add it to worklist to be revisited.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46661