Be conservative about allocations that may alias the accessed pointer.
If an allocation has a must-alias relation to the access pointer, we treat it
as a Def. Otherwise, without this check, the code here was just skipping over
the allocation call and ignoring it. I noticed this by inspection and don't
have a specific testcase that it breaks, but it seems like we need to treat
a may-alias allocation as a Clobber.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163127
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