[InstSimplify] add tests for possible sdiv/srem simplifications; NFC

[InstSimplify] add tests for possible sdiv/srem simplifications; NFC

As noted in PR34517, the handling of signed div/rem is not on par with
unsigned div/rem. Signed is harder to reason about, but it should be
possible to handle at least some of these using the same technique that
we use for unsigned: use icmp logic to see if there's a relationship
between the quotient and divisor.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312938 91177308-0d34-0410-b5e6-96231b3b80d8