+Note [Tidy IdInfo]
+~~~~~~~~~~~~~~~~~~
+All nested Ids now have the same IdInfo, namely vanillaIdInfo, which
+should save some space; except that we preserve occurrence info for
+two reasons:
+
+ (a) To make printing tidy core nicer
+
+ (b) Because we tidy RULES and InlineRules, which may then propagate
+ via --make into the compilation of the next module, and we want
+ the benefit of that occurrence analysis when we use the rule or
+ or inline the function. In particular, it's vital not to lose
+ loop-breaker info, else we get an infinite inlining loop
+
+Note that tidyLetBndr puts more IdInfo back.
+
+