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

1 files changed, 5 insertions, 5 deletions

diff --git a/testsuite/tests/simplCore/should_compile/T3772.stdout b/testsuite/tests/simplCore/should_compile/T3772.stdout
index 44aee7b69e..a4ab97da38 100644
--- a/testsuite/tests/simplCore/should_compile/T3772.stdout
+++ b/testsuite/tests/simplCore/should_compile/T3772.stdout
@@ -1,7 +1,7 @@
 
 ==================== Tidy Core ====================
 Result size of Tidy Core
-  = {terms: 44, types: 19, coercions: 0, joins: 0/0}
+  = {terms: 43, types: 18, coercions: 0, joins: 0/0}
 
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 T3772.$trModule4 :: GHC.Prim.Addr#
@@ -59,14 +59,14 @@ $wxs
       }
 end Rec }
 
--- RHS size: {terms: 11, types: 3, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 10, types: 2, coercions: 0, joins: 0/0}
 T3772.$wfoo [InlPrag=NOINLINE] :: GHC.Prim.Int# -> ()
 [GblId, Arity=1, Caf=NoCafRefs, Str=<S,U>]
 T3772.$wfoo
   = \ (ww :: GHC.Prim.Int#) ->
-      case GHC.Prim.tagToEnum# @ Bool (GHC.Prim.<# 0# ww) of {
-        False -> GHC.Tuple.();
-        True -> $wxs ww
+      case GHC.Prim.<# 0# ww of {
+        __DEFAULT -> GHC.Tuple.();
+        1# -> $wxs ww
       }
 
 -- RHS size: {terms: 6, types: 3, coercions: 0, joins: 0/0}