[InstSimplify] restrict icmp fold with 2 sdiv exact operands (PR32949)

These folds were introduced with https://reviews.llvm.org/rL127064 as part of solving:
https://bugs.llvm.org/show_bug.cgi?id=9343

As shown here:
http://rise4fun.com/Alive/C8
...however, the sdiv exact case needs a stronger predicate.

I opted for duplicated code instead of adding another fallthrough because I think that's 
easier to read (and edit in case we need/want to restrict/loosen the predicates any more).

This should fix:
https://bugs.llvm.org/show_bug.cgi?id=32949
https://bugs.llvm.org/show_bug.cgi?id=32948

Differential Revision: https://reviews.llvm.org/D32954

llvm-svn: 303104