Re-engineer caseRules to add tagToEnum/dataToTag

See Note [Scrutinee Constant Folding] in SimplUtils

* Add cases for tagToEnum and dataToTag. This is the main new
  bit.  It allows the simplifier to remove the pervasive uses
  of     case tagToEnum (a > b) of
            False -> e1
            True  -> e2
  and replace it by the simpler
         case a > b of
            DEFAULT -> e1
            1#      -> e2
  See Note [caseRules for tagToEnum]
  and Note [caseRules for dataToTag] in PrelRules.

* This required some changes to the API of caseRules, and hence
  to code in SimplUtils.  See Note [Scrutinee Constant Folding]
  in SimplUtils.

* Avoid duplication of work in the (unusual) case of
     case BIG + 3# of b
       DEFAULT -> e1
       6#      -> e2

  Previously we got
     case BIG of
       DEFAULT -> let b = BIG + 3# in e1
       3#      -> let b = 6#       in e2

  Now we get
     case BIG of b#
       DEFAULT -> let b = b' + 3# in e1
       3#      -> let b = 6#      in e2

* Avoid duplicated code in caseRules

A knock-on refactoring:

* Move Note [Word/Int underflow/overflow] to Literal, as
  documentation to accompany mkMachIntWrap etc; and get
  rid of PrelRuls.intResult' in favour of mkMachIntWrap

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