Start documenting missed optimisation

Author: chambart

doc/missed_optimisations.md

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+# List of missed optimisations to look at later
+
+This should be compiled to the identity. But the CSE environment seems to lose
+the relationship in the join after the first switch.
+
+   let f x =
+     let not_x = if x then false else true in
+     if not_x then false else true