[AArch64] Teach AArch64TargetLowering::getOptimalMemOpType to consider alignment

[AArch64] Teach AArch64TargetLowering::getOptimalMemOpType to consider alignment
restrictions when choosing a type for small-memcpy inlining in
SelectionDAGBuilder.

This ensures that the loads and stores output for the memcpy won't be further
expanded during legalization, which would cause the total number of instructions
for the memcpy to exceed (often significantly) the inlining thresholds.

<rdar://problem/17829180>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234462 91177308-0d34-0410-b5e6-96231b3b80d8