Define chkAppend, and use it

Somtimes we need (xs ++ ys) in situations where ys is
almost always empty.  Utils.chkAppend checks for that
case first.

compiler/types/FamInstEnv.lhs

@@ -754,13 +754,11 @@ find match_fun match_tys (inst@(FamInst { fi_branches = branches, fi_branched =
               where 
                 match = FamInstMatch { fim_instance = inst
                                      , fim_index    = ind
-                                     , fim_tys      = substTyVars subst tvs `add_on` match_tys2}
+                                     , fim_tys      = substTyVars subst tvs `chkAppend` match_tys2}
       where
         -- Deal with over-saturation
         -- See Note [Over-saturated matches]
         (match_tys1, match_tys2) = splitAtList mb_tcs match_tys
-        add_on tys1 []   = tys1  -- The wildly common case
-        add_on tys1 tys2 = tys1 ++ tys2
 
 lookup_fam_inst_env           -- The worker, local to this module
     :: MatchFun

compiler/utils/Util.lhs

@@ -18,7 +18,7 @@ module Util (
 
         unzipWith,
 
-        mapFst, mapSnd,
+        mapFst, mapSnd, chkAppend,
         mapAndUnzip, mapAndUnzip3, mapAccumL2,
         nOfThem, filterOut, partitionWith, splitEithers,
 
@@ -259,6 +259,13 @@ splitEithers (e : es) = case e of
                         Left x -> (x:xs, ys)
                         Right y -> (xs, y:ys)
     where (xs,ys) = splitEithers es
+
+chkAppend :: [a] -> [a] -> [a]
+-- Checks for the second arguemnt being empty
+-- Used in situations where that situation is common
+chkAppend xs ys 
+  | null ys   = xs
+  | otherwise = xs ++ ys
 \end{code}
 
 A paranoid @zip@ (and some @zipWith@ friends) that checks the lists