In D77454 we explain that LoadInst and StoreInst always have their alignment defined.
This allows to work backward here and to infer that getNewAlignment does not need to return 0 in case of failure.
Returning 1 also works since it needs to be greater than the Load/Store alignment which is a least 1.
This is patch is part of a series to introduce an Alignment type.
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2019-July/133851.html
See this patch for the introduction of the type: https://reviews.llvm.org/D64790