[DivRemPairs] Add srem-of-srem tests (PR42823, D65298, D65451)

The @srem_of_srem_expanded case exposed a RAUW pitfall in D65298.
Right now these don't appear to fail verification,
so it should be safe to precommit them.

https://reviews.llvm.org/D65298
https://bugs.llvm.org/show_bug.cgi?id=42823
https://reviews.llvm.org/D65451

llvm-svn: 367325