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I believe originally wasm did not allow overlapping segments, that is, where
one memory segment tramples the data from a previous one. But then the
spec changed its mind and we allowed it. Binaryen seems to have assumed
the original case, and not checked for trampling.
If there is a chance of trampling, we cannot optimize out zeros - the zero
may have an effect if it tramples data from a previous segment. This does
not occur in practice in LLVM output, which is why this wasn't a problem
so far, I think.
An existing testcase hit this issue, so I split it up.
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