[Alignment][NFC] Assume AlignmentFromAssumptions::getNewAlignment is always set.

Summary:
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

Reviewers: courbet

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77538