[BasicAA] Don't call isKnownNonEqual if we might be have gone through a PHINode.

This is a fix for the test case in PR32314.

Basic Alias Analysis can ask if two nodes are known non-equal after looking through a phi node to find a GEP. isAddOfNonZero saw an add of a constant from the same phi and said that its output couldn't be equal. But Basic Alias Analysis was really asking about the value from the previous loop iteration.

This patch at least makes that case not happen anymore, I'm not sure if there were still other ways this can fail. As was discussed in the bug, it looks like fixing BasicAA would be difficult so this patch seemed like a possible workaround

Differential Revision: https://reviews.llvm.org/D33136

llvm-svn: 305481