Force imposs_deflt_cons in filterAlts

This fixes a pretty serious space leak as the forced thunk would retain
`Alt b` values which would then contain reference to a lot of old
bindings and other simplifier gunk.

The OtherCon unfolding was not forced on subsequent simplifier runs so
more and more old stuff would be retained until the end of
simplification.

Fixing this has a drastic effect on maximum residency for the mmark
package which goes from

```
  45,005,401,056 bytes allocated in the heap
  17,227,721,856 bytes copied during GC
     818,281,720 bytes maximum residency (33 sample(s))
       9,659,144 bytes maximum slop
            2245 MiB total memory in use (0 MB lost due to fragmentation)
```

to

```
  45,039,453,304 bytes allocated in the heap
  13,128,181,400 bytes copied during GC
     331,546,608 bytes maximum residency (40 sample(s))
       7,471,120 bytes maximum slop
             916 MiB total memory in use (0 MB lost due to fragmentation)
```

See #21993 for some more discussion.

compiler/GHC/Core/Utils.hs

@@ -706,7 +706,7 @@ filterAlts :: TyCon                -- ^ Type constructor of scrutinee's type (us
              -- in a "case" statement then they will need to manually add a dummy case branch that just
              -- calls "error" or similar.
 filterAlts _tycon inst_tys imposs_cons alts
-  = (imposs_deflt_cons, addDefault trimmed_alts maybe_deflt)
+  = imposs_deflt_cons `seqList` (imposs_deflt_cons, addDefault trimmed_alts maybe_deflt)
   where
     (alts_wo_default, maybe_deflt) = findDefault alts
     alt_cons = [con | Alt con _ _ <- alts_wo_default]
@@ -714,7 +714,7 @@ filterAlts _tycon inst_tys imposs_cons alts
     trimmed_alts = filterOut (impossible_alt inst_tys) alts_wo_default
 
     imposs_cons_set = Set.fromList imposs_cons
-    imposs_deflt_cons =
+    !imposs_deflt_cons =
       imposs_cons ++ filterOut (`Set.member` imposs_cons_set) alt_cons
          -- "imposs_deflt_cons" are handled
          --   EITHER by the context,