Give packHalfWordsCLit a more specific type

I'm not sure if there's a reason why the HeapRep constructor takes
2 WordOffs rather than 2 StgHalfWords.

compiler/cmm/CmmInfo.hs

@@ -183,7 +183,7 @@ mkInfoTableContents dflags
        ; return (prof_data ++ liveness_data, (std_info, srt_label)) }
 
   | HeapRep _ ptrs nonptrs closure_type <- smrep
-  = do { let layout  = packHalfWordsCLit dflags ptrs nonptrs
+  = do { let layout  = packHalfWordsCLit dflags (fromIntegral ptrs) (fromIntegral nonptrs)
        ; (prof_lits, prof_data) <- mkProfLits dflags prof
        ; let (srt_label, srt_bitmap) = mkSRTLit dflags srt
        ; (mb_srt_field, mb_layout, extra_bits, ct_data)

compiler/cmm/CmmUtils.hs

@@ -158,7 +158,7 @@ mkRODataLits lbl lits
 mkWordCLit :: DynFlags -> StgWord -> CmmLit
 mkWordCLit dflags wd = CmmInt (fromIntegral wd) (wordWidth dflags)
 
-packHalfWordsCLit :: (Integral a, Integral b) => DynFlags -> a -> b -> CmmLit
+packHalfWordsCLit :: DynFlags -> StgHalfWord -> StgHalfWord -> CmmLit
 -- Make a single word literal in which the lower_half_word is
 -- at the lower address, and the upper_half_word is at the
 -- higher address