[clang] [llvm] [IR] Optimize PHINode::removeIncomingValue() by swapping removed incoming value with the last incoming value. (PR #171963)

[Walter Lee via cfe-commits]

googlewalt wrote:

Hi.  We are seeing that this causes non-determinism in some of our internal compiles.  We're trying to get a standalone reproducer, but currently the reproducer is quite complex (involving FDO).

https://github.com/llvm/llvm-project/pull/171963