+Note [Add scrutinee to ValueEnv too]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this:
+ case x of y
+ (a,b) -> case b of c
+ I# v -> ...(f y)...
+By the time we get to the call (f y), the ValueEnv
+will have a binding for y, and for c
+ y -> (a,b)
+ c -> I# v
+BUT that's not enough! Looking at the call (f y) we
+see that y is pair (a,b), but we also need to know what 'b' is.
+So in extendCaseBndrs we must *also* add the binding
+ b -> I# v
+else we lose a useful specialisation for f. This is necessary even
+though the simplifier has systematically replaced uses of 'x' with 'y'
+and 'b' with 'c' in the code. The use of 'b' in the ValueEnv came
+from outside the case. See Trac #4908 for the live example.
+