test for promotion in compound assignment

The C frontend needs to apply the usual arithmetic promotion rules in
compound assignments. Issue #3706.

Author: Daniel Kroening

regression/cbmc/compound-assignment/compound_promotion.c

@@ -0,0 +1,10 @@
+#include <assert.h>
+
+int main()
+{
+  unsigned char x = 50;
+  short y = -5;
+  x /= y;
+  assert(x == (unsigned char)-10);
+  return 0;
+}

regression/cbmc/compound-assignment/compound_promotion.desc

@@ -0,0 +1,11 @@
+KNOWNBUG
+compound_promition.c
+
+^EXIT=0$
+^SIGNAL=0$
+^VERIFICATION SUCCESSFUL$
+--
+^warning: ignoring
+--
+The C frontend needs to apply the usual arithmetic promotion rules in
+compound assignments. Issue #3706.