RegAlloc: Make some pattern matched complete

these actually are complete, but due to the use of pattern guards, the
compiler does not see that. Refactor the code that it does.

Differential Revision: https://phabricator.haskell.org/D2574

compiler/nativeGen/RegAlloc/Graph/Main.hs

@@ -360,6 +360,9 @@ graphAddCoalesce (r1, r2) graph
         , RegReal _             <- r2
         = graph
 
+        | otherwise
+        = panic "graphAddCoalesce"
+
 
 -- | Patch registers in code using the reg -> reg mapping in this graph.
 patchRegsFromGraph

compiler/nativeGen/RegAlloc/Liveness.hs

@@ -897,12 +897,9 @@ livenessForward
 
 livenessForward _        _           []  = []
 livenessForward platform rsLiveEntry (li@(LiveInstr instr mLive) : lis)
-        | Nothing               <- mLive
-        = li : livenessForward platform rsLiveEntry lis
-
         | Just live <- mLive
-        , RU _ written  <- regUsageOfInstr platform instr
         = let
+                RU _ written  = regUsageOfInstr platform instr
                 -- Regs that are written to but weren't live on entry to this instruction
                 --      are recorded as being born here.
                 rsBorn          = mkUniqSet
@@ -915,6 +912,9 @@ livenessForward platform rsLiveEntry (li@(LiveInstr instr mLive) : lis)
         in LiveInstr instr (Just live { liveBorn = rsBorn })
                 : livenessForward platform rsLiveNext lis
 
+        | otherwise
+        = li : livenessForward platform rsLiveEntry lis
+
 
 -- | Calculate liveness going backwards,
 --   filling in when regs die, and what regs are live across each instruction