COFF: ICF should not merge sectinos if their alignments are not the same.

There's actually a room to improve this patch. Instead of not merging
sections that have different alignements, we can choose the section that
has the largest alignment requirement among all sections that are otherwise
considered the same. Then all section alignments are satisfied, so we can
merge them.

I don't know if that improvement could make any difference for real-world
input, so I'll leave it alone. Would be interesting to revisit later.

llvm-svn: 248581