[InstSimplify][ConstantFolding] Teach constant folding how to handle icmp null, ...

[InstSimplify][ConstantFolding] Teach constant folding how to handle icmp null, (inttoptr x) as well as it handles icmp (inttoptr x), null

Summary:
The constant folding code currently assumes that the constant expression will always be on the left and the simple null will be on the right. But that's not true at least on the path from InstSimplify.

This patch adds support to ConstantFolding to detect the reversed case.

Reviewers: spatel, dberlin, majnemer, davide, joey

Reviewed By: joey

Subscribers: joey, llvm-commits

Differential Revision: https://reviews.llvm.org/D33801

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304559 91177308-0d34-0410-b5e6-96231b3b80d8