InstCombine: Don't turn -(x/INT_MIN) -> x/INT_MIN

It is not safe to negate the smallest signed integer, doing so yields
the same number back.

This fixes PR20186.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212164 91177308-0d34-0410-b5e6-96231b3b80d8