add unreachable based on invariants

compiler/builtins/bitcode/src/dec.zig

@@ -308,7 +308,9 @@ pub const RocDec = extern struct {
 
         // (n / 0) is an error
         if (denominator_i128 == 0) {
-            @panic("TODO runtime exception for divide by 0!");
+            // The compiler frontend does the `denominator == 0` check for us,
+            // therefore this case is unreachable from roc user code
+            unreachable;
         }
 
         // If they're both negative, or if neither is negative, the final answer