Revise alignment checking/calculation on 256-bit unaligned memory access

Revise alignment checking/calculation on 256-bit unaligned memory access

- It's still considered aligned when the specified alignment is larger
  than the natural alignment;
- The new alignment for the high 128-bit vector should be min(16,
  alignment) as the pointer is advanced by 16, a power-of-2 offset.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177947 91177308-0d34-0410-b5e6-96231b3b80d8