[LLVMdev] Intended semantics for ``fence seq_cst``

[JF Bastien]

> You will need to do this in the frontend. The target independent
optimizers are allowed to use the memory model.

We discussed doing this, and concluded that doing it pre-opt was overly
restrictive on correct code. Doing it post-opt bakes the behavior into the
portable code, so in a way it'll be reliably broken but won't penalize good
code.

FWIW it's easy to change from one to the other: move one line of code. I
hope my explanation makes sense, and it doesn't look like I'm dismissing
your comment on implementation issues.
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