test/Simplify: don't crash on divide by zero

Currently test/Simplify would not check for 0 before trying to do
constant folding, which is quite bad since it simply crashes the
program.

The fix is to simply avoid evaluating the expression if there is a
division by zero.

testing/Main.hs

@@ -32,6 +32,7 @@ goldensTests = Framework.testGroup "Goldens tests"
         , "test4"
         , "test5"
         , "test6"
+        , "test7"
         , "if-test"
         , "if-test2"
         , "if-test3"

testing/Simplify.hs

@@ -27,6 +27,8 @@ simplify = deepFwdRw simp
     = Just $ Lit $ Int $ n1 + n2
     -- ... more cases for constant folding
 -- @ end cprop.tex
+  s_exp (Binop Div _lhs (Lit (Int 0)))
+    = Nothing
   s_exp (Binop opr e1 e2)
     | (Just op, Lit (Int i1), Lit (Int i2)) <- (intOp opr, e1, e2) =
         Just $ Lit $ Int  $ op i1 i2

testing/tests/test7

+-- Test that we don't crash on divide by zero.
+procName() {
+L0:
+  reg1 = 42 / 0
+  ret(reg1)
+}

testing/tests/test7.expected

+procName() {
+L0:
+  reg1 = 42 / 0
+  ret(reg1)
+}