Fix comparisons of alloca alignment in inliner merging

Duncan pointed out a mistake in my fix in r186425 when only one of the allocas
being compared had the target-default alignment. This is essentially his
suggested solution. Thanks!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186510 91177308-0d34-0410-b5e6-96231b3b80d8