Fix a bug with block layout when the block contains something

Fix a bug with block layout when the block contains something
more aligned than the block header but also contains something
smaller than the block-header alignment but not exactly half
the difference between the large alignment and the header
alignment.  Got that?

I'm really not sure what I was thinking with the buggy computation
here, but the fix is pretty obvious.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155662 91177308-0d34-0410-b5e6-96231b3b80d8