Add as-if rule article (#29)

wiki_articles/as_if.md

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+# What Is the As-If Rule, and How Does the Compiler Optimize Code?
+
+The compiler can transform code however it thinks is best, as long as this
+doesn't change the *observable behavior* of the program.
+People call this the *[as-if rule][as-if]*.
+Observable behavior includes, but is not limited to:
+- writing output to the terminal (**[printf][printf]**, **[std::cout][cout]**, etc.)
+- aborting the program (**[exit][exit]**, **[std::terminate][term]**, etc.)
+- access of **[volatile][volatile]** objects
+
+<!-- inline -->
+## Example
+```cpp
+int main() {
+  if (false)
+    printf("unreachable");
+  int x = 3;
+  return x + 7;
+}
+```
+The *observable behavior* is that the program returns the exit code `10`.
+
+<!-- inline -->
+## Assembly Output
+```x86asm
+main:
+; 'if (false)' removed by
+; dead-code elimination.
+; 'x' is constant-folded.
+  mov eax, 10
+  ret
+```
+*See [live example at Compiler Explorer][ce]*
+
+[as-if]: https://en.cppreference.com/w/cpp/language/as_if
+[volatile]: https://en.cppreference.com/w/cpp/language/cv
+[printf]: https://en.cppreference.com/w/c/io/fprintf
+[cout]: https://en.cppreference.com/w/cpp/io/cout
+[exit]: https://en.cppreference.com/w/c/program/exit
+[term]: https://en.cppreference.com/w/cpp/error/terminate
+[ce]: https://godbolt.org/z/eTbv3458a