Use the target-aware constant folder on expressions to improve the chance

Use the target-aware constant folder on expressions to improve the chance
they'll be simple enough to simulate, and to reduce the chance we'll encounter
equal but different simple pointer constants.

This removes the symptoms from PR11352 but is not a full fix. A proper fix would
either require a guarantee that two constant objects we simulate are folded
when equal, or a different way of handling equal pointers (ie., trying a
constantexpr icmp on them to see whether we know they're equal or non-equal or
unsure).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151093 91177308-0d34-0410-b5e6-96231b3b80d8