Fix Alpine's assert-statement conversion special case

In d44bfd3 we added a special case to catch Alpine Linux' assert
structure. The match, however, was not sufficiently specific as it just
accepted any statement that follows the actual assertion.

Fixes: #8436

regression/ansi-c/goto_convert_assert/main.c

@@ -0,0 +1,22 @@
+void __assert_fail(char *, char *, unsigned, char *);
+
+int main()
+{
+  (void)((1 < 2) || (__CPROVER_assert(0, ""), 0));
+
+  int jumpguard;
+  jumpguard = (jumpguard | 1);
+label_1:;
+  {
+    while(1)
+    {
+      if(jumpguard == 0)
+      {
+        __assert_fail("0", "lc2.c", 8U, "func");
+        goto label_1;
+      }
+      goto label_2;
+    }
+  label_2:;
+  }
+}

regression/ansi-c/goto_convert_assert/test.desc

@@ -0,0 +1,7 @@
+CORE test-c++-front-end
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring

src/ansi-c/goto-conversion/goto_convert.cpp

@@ -1812,8 +1812,11 @@ void goto_convertt::generate_ifthenelse(
   if(
     is_empty(false_case) && true_case.instructions.size() == 2 &&
     true_case.instructions.front().is_assert() &&
-    true_case.instructions.front().condition().is_false() &&
+    simplify_expr(true_case.instructions.front().condition(), ns).is_false() &&
     true_case.instructions.front().labels.empty() &&
+    true_case.instructions.back().is_other() &&
+    true_case.instructions.back().get_other().get_statement() ==
+      ID_expression &&
     true_case.instructions.back().labels.empty())
   {
     true_case.instructions.front().condition_nonconst() = boolean_negate(guard);