Tag inference work.

This does three major things:
* Enforce the invariant that all strict fields must contain tagged
pointers.
* Try to predict the tag on bindings in order to omit tag checks.
* Allows functions to pass arguments unlifted (call-by-value).

The former is "simply" achieved by wrapping any constructor allocations with
a case which will evaluate the respective strict bindings.

The prediction is done by a new data flow analysis based on the STG
representation of a program. This also helps us to avoid generating
redudant cases for the above invariant.

StrictWorkers are created by W/W directly and SpecConstr indirectly.
See the Note [Strict Worker Ids]

Other minor changes:

* Add StgUtil module containing a few functions needed by, but
  not specific to the tag analysis.

-------------------------
Metric Decrease:
	T12545
	T18698b
	T18140
	T18923
        LargeRecord
Metric Increase:
        LargeRecord
	ManyAlternatives
	ManyConstructors
	T10421
	T12425
	T12707
	T13035
	T13056
	T13253
	T13253-spj
	T13379
	T15164
	T18282
	T18304
	T18698a
	T1969
	T20049
	T3294
	T4801
	T5321FD
	T5321Fun
	T783
	T9233
	T9675
	T9961
	T19695
	WWRec
-------------------------

4 files changed, 15 insertions, 3 deletions

diff --git a/compiler/GHC/Tc/Module.hs b/compiler/GHC/Tc/Module.hs
index 026686b8cc..d9b59b4fd8 100644
--- a/compiler/GHC/Tc/Module.hs
+++ b/compiler/GHC/Tc/Module.hs
@@ -2975,6 +2975,7 @@ ppr_types debug type_env
       | otherwise = hasTopUserName id
                     && case idDetails id of
                          VanillaId    -> True
+                         StrictWorkerId{} -> True
                          RecSelId {}  -> True
                          ClassOpId {} -> True
                          FCallId {}   -> True
diff --git a/compiler/GHC/Tc/Solver.hs b/compiler/GHC/Tc/Solver.hs
index 6a1f2d3315..8b6ac9928d 100644
--- a/compiler/GHC/Tc/Solver.hs
+++ b/compiler/GHC/Tc/Solver.hs
@@ -677,7 +677,7 @@ type-class or type defined in N.
 Secondly, when should these heuristics be enforced? We enforced them when the
 type-class method call site is in a module marked `-XSafe` or `-XTrustworthy`.
 This allows `-XUnsafe` modules to operate without restriction, and for Safe
-Haskell inferrence to infer modules with unsafe overlaps as unsafe.
+Haskell inference to infer modules with unsafe overlaps as unsafe.
 
 One alternative design would be to also consider if an instance was imported as
 a `safe` import or not and only apply the restriction to instances imported
@@ -745,7 +745,7 @@ How is this implemented? It's complicated! So we'll step through it all:
       IORefs called `tcg_safe_infer` and `tcg_safe_infer_reason`.
 
  7) `GHC.Driver.Main.tcRnModule'` -- Reads `tcg_safe_infer` after type-checking, calling
-    `GHC.Driver.Main.markUnsafeInfer` (passing the reason along) when safe-inferrence
+    `GHC.Driver.Main.markUnsafeInfer` (passing the reason along) when safe-inference
     failed.
 
 Note [No defaulting in the ambiguity check]
diff --git a/compiler/GHC/Tc/Types.hs b/compiler/GHC/Tc/Types.hs
index 97f11c8a0b..e1f0400e44 100644
--- a/compiler/GHC/Tc/Types.hs
+++ b/compiler/GHC/Tc/Types.hs
@@ -50,7 +50,7 @@ module GHC.Tc.Types(
         PromotionErr(..),
         IdBindingInfo(..), ClosedTypeId, RhsNames,
         IsGroupClosed(..),
-        SelfBootInfo(..),
+        SelfBootInfo(..), bootExports,
         tcTyThingCategory, pprTcTyThingCategory,
         peCategory, pprPECategory,
         CompleteMatch, CompleteMatches,
@@ -696,6 +696,15 @@ data SelfBootInfo
 -- What is sb_tcs used for?  See Note [Extra dependencies from .hs-boot files]
 -- in GHC.Rename.Module
 
+bootExports :: SelfBootInfo -> NameSet
+bootExports boot =
+  case boot of
+    NoSelfBoot -> emptyNameSet
+    SelfBoot { sb_mds = mds} ->
+      let exports = md_exports mds
+      in availsToNameSet exports
+
+
 
 {- Note [Tracking unused binding and imports]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/compiler/GHC/Tc/Types.hs-boot b/compiler/GHC/Tc/Types.hs-boot
index c6302adb57..9f107936a5 100644
--- a/compiler/GHC/Tc/Types.hs-boot
+++ b/compiler/GHC/Tc/Types.hs-boot
@@ -6,6 +6,8 @@ import GHC.Utils.Outputable
 
 data TcLclEnv
 
+data SelfBootInfo
+
 data TcIdSigInfo
 instance Outputable TcIdSigInfo