[ValueTracking] Handle non-pow2 align assume bundle (PR53693)

https://reviews.llvm.org/D119414 clarified that this is legal IR,
so handle it gracefully. (We could aggressively use the fact that
the pointer must be a null pointer in that case, but I'm not
bothering with that.)

Fixes https://github.com/llvm/llvm-project/issues/53693.