Emit atomic-fetches that compare against negated addends

[MattWindsor91]

A variant of #237 concerns rewriting things like (C syntax, but this happens at the x86 level)

if (atomic_fetch_add(x, y) > -y) {
   // ...
}

into something like

if (lock(x -= -y) > -y) {
  // ...
}

(and similarly for atomic_fetch_sub(x, -y)) with the idea that the locked subtract sets up the same flags on x86 that we'd use for the comparison anyway. We don't seem to hit this optimisation yet, so it might be worth having comparison generators that trip it.

[MattWindsor91]

I think this would become a new if action set, something like:

• Find non-dependent atomic variable x with KV k
• Make sure k isn't INT_MIN
• Emit if (atomic_fetch_sub(x, k) > k) { /* dead code */ }, or if (atomic_fetch_sub(x, k) >= k) { / surround / }, etc. Maybe have a flag that flicks between sub and add-negative, and maybe have a selection of relational operators that have/don't have equality inside.

Probably a set of actions that look like this:

• if.surround.kv-sub
• if.insert.dead.kv-sub (we don't have any deadcode if insertions yet, so the naming is speculative)