+Note [Cannot trivialise]
+~~~~~~~~~~~~~~~~~~~~~~~~
+Consider tih
+ f :: Int -> Addr#
+
+ foo :: Bar
+ foo = Bar (f 3)
+
+Then we can't ANF-ise foo, even though we'd like to, because
+we can't make a top-level binding for the Addr# (f 3). And if
+so we don't want to turn it into
+ foo = let x = f 3 in Bar x
+because we'll just end up inlining x back, and that makes the
+simplifier loop. Better not to ANF-ise it at all.
+
+A case in point is literal strings (a MachStr is not regarded as
+trivial):
+
+ foo = Ptr "blob"#
+
+We don't want to ANF-ise this.