This is a redo of D7208 ( r227242 - http://llvm.org/viewvc/llvm-project?view=revision&revision=227242 ).
The patch was reverted because an AArch64 target could infinite loop after the change in DAGCombiner to merge vector stores. That happened because AArch64's allowsMisalignedMemoryAccesses() wasn't telling the truth. It reported all unaligned memory accesses as fast, but then split some 128-bit unaligned accesses up in performSTORECombine() because they are slow.
This patch attempts to fix the problem in allowsMisalignedMemoryAccesses() while preserving existing lowering behavior for AArch64.