Fix GHC warnings.

Fix all warnings except `-Wincomplete-uni-patterns` and `-Wincomplete-record-updates`,
which were newly added to `-Wall` (starting in GHC 9.2.1).

https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0071-Wall-uni-patterns.rst

Author: dan-zheng

src/lib/ImpToLLVM.hs

@@ -481,6 +481,7 @@ compileInstr instr = case instr of
                  Scalar Word64Type -> False
                  Scalar Float64Type -> True
                  Scalar Float32Type -> True
+                 Scalar _ -> error "Unknown scalar type"
                  Vector _ ty' -> signed (Scalar ty')
                  PtrType _ -> False
                int_to_float = if signed (getIType ix) then L.SIToFP else L.UIToFP

src/lib/Inference.hs

@@ -20,7 +20,7 @@ import Control.Monad.State.Strict
 import Control.Monad.Writer.Strict hiding (Alt)
 import Control.Monad.Reader
 import Data.Coerce
-import Data.Foldable (toList, asum)
+import Data.Foldable (toList)
 import Data.Functor ((<&>))
 import Data.List (sortOn)
 import Data.Maybe (fromJust, fromMaybe, catMaybes)

src/lib/LLVM/Compile.hs

@@ -28,8 +28,6 @@ import qualified Data.ByteString       as BS
 import qualified Data.ByteString.Char8 as B
 import System.IO.Unsafe
 
-import Control.Monad
-
 import Logging
 import PPrint ()
 import Paths_dex  (getDataFileName)

src/lib/RenderHtml.hs

@@ -99,7 +99,6 @@ syntaxSpan s c = H.span (toHtml s) ! class_ (stringValue className)
       SymbolStr   -> "symbol"
       TypeNameStr -> "type-name"
       IsoSugarStr -> "iso-sugar"
-      NormalStr -> error "Should have been matched already"
 
 data StrClass = NormalStr
               | CommentStr | KeywordStr | CommandStr | SymbolStr | TypeNameStr