[llvm] [BasicAA] Analyze constant offsets through select (PR #218203)

[via llvm-commits]

SomeFlyingThing wrote:

> No impact on llvm-opt-benchmark. What is the real-world motivation for this?

fixes a missed BasicAA case that was literally already marked with a TODO: should be "NoAlias" here as well

https://github.com/llvm/llvm-project/pull/218203