Relax unaligned access assertion when type is byte aligned

Summary:
This relaxes an assertion inside SelectionDAGBuilder which is overly
restrictive on targets which have no concept of alignment (such as AVR).

In these architectures, all types are aligned to 8-bits.

After this, LLVM will only assert that accesses are aligned on targets
which actually require alignment.

This patch follows from a discussion on llvm-dev a few months ago
http://llvm.1065342.n5.nabble.com/llvm-dev-Unaligned-atomic-load-store-td112815.html

Reviewers: bogner, nemanjai, joerg, efriedma

Reviewed By: efriedma

Subscribers: efriedma, cactus, llvm-commits

Differential Revision: https://reviews.llvm.org/D39946

llvm-svn: 320243