when merging two alias sets, the result set is volatile if either

when merging two alias sets, the result set is volatile if either
of the sets is volatile.  We were dropping the volatile bit of the
merged in set, leading (luckily) to assertions in cases like
PR7535.  I cannot produce a testcase that repros with opt, but this
is obviously correct.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112402 91177308-0d34-0410-b5e6-96231b3b80d8