Enhance cast worker/wrapper for INLINABLE

In #19890 we realised that cast worker/wrapper didn't really work
properly for functions with an INLINABLE pragma, and hence a stable
unfolding. This patch fixes the problem.

Instead of disabling cast w/w when there is a stable unfolding (as
we did before), we now tranfer the stable unfolding to the worker.

It turned out that it was easier to do that if I moved the cast
w/w stuff from prepareBinding to completeBind.

No chnages at all in nofib results:
--------------------------------------------------------------------------------
        Program           Size    Allocs   Runtime   Elapsed  TotalMem
--------------------------------------------------------------------------------
            Min          -0.0%      0.0%    -63.8%    -78.2%      0.0%
            Max          -0.0%      0.0%    +11.8%    +11.7%      0.0%
 Geometric Mean          -0.0%     -0.0%    -26.6%    -33.4%     -0.0%

Small decreases in compile-time allocation for two tests (below)
of around 2%.

T12545 increased in compile-time alloc by 4%, but it's not
reproducible on my machine, and is a known-wobbly test.

Metric Increase:
    T12545

Metric Decrease:
    T18698a
    T18698b

1 files changed, 18 insertions, 2 deletions

diff --git a/compiler/GHC/Core.hs b/compiler/GHC/Core.hs
index 05df4a7a7d..a17604300f 100644
--- a/compiler/GHC/Core.hs
+++ b/compiler/GHC/Core.hs
@@ -64,8 +64,8 @@ module GHC.Core (
         maybeUnfoldingTemplate, otherCons,
         isValueUnfolding, isEvaldUnfolding, isCheapUnfolding,
         isExpandableUnfolding, isConLikeUnfolding, isCompulsoryUnfolding,
-        isStableUnfolding, hasCoreUnfolding, hasSomeUnfolding,
-        isBootUnfolding,
+        isStableUnfolding, isInlineUnfolding, isBootUnfolding,
+        hasCoreUnfolding, hasSomeUnfolding,
         canUnfold, neverUnfoldGuidance, isStableSource,
 
         -- * Annotated expression data types
@@ -1462,6 +1462,22 @@ isStableUnfolding (CoreUnfolding { uf_src = src }) = isStableSource src
 isStableUnfolding (DFunUnfolding {})               = True
 isStableUnfolding _                                = False
 
+isInlineUnfolding :: Unfolding -> Bool
+-- ^ True of a /stable/ unfolding that is
+--   (a) always inlined; that is, with an `UnfWhen` guidance, or
+--   (b) a DFunUnfolding which never needs to be inlined
+isInlineUnfolding (CoreUnfolding { uf_src = src, uf_guidance = guidance })
+  | isStableSource src
+  , UnfWhen {} <- guidance
+  = True
+
+isInlineUnfolding (DFunUnfolding {})
+  = True
+
+-- Default case
+isInlineUnfolding _ = False
+
+
 -- | Only returns False if there is no unfolding information available at all
 hasSomeUnfolding :: Unfolding -> Bool
 hasSomeUnfolding NoUnfolding   = False