Modcc/optimise divide by const (#1043)

Add a simple re-write to eliminate division by a known constant.

modcc/symdiff.cpp

@@ -430,6 +430,9 @@ public:
         else if (expr_value(rhs)==1) {
             result_ = e->lhs()->clone();
         }
+        else if (is_number(rhs)) {
+            result_ = make_expression<MulBinaryExpression>(loc, std::move(lhs), make_expression<NumberExpression>(loc, 1.0/expr_value(rhs)));
+        }
         else {
             result_ = make_expression<DivBinaryExpression>(loc, std::move(lhs), std::move(rhs));
         }

test/unit-modcc/test_printers.cpp

@@ -200,7 +200,7 @@ TEST(CPrinter, proc_body_const) {
 
 TEST(CPrinter, proc_body_inlined) {
     const char* expected =
-        "r_9_=s2[i_]/3;\n"
+        "r_9_=s2[i_]*0.33333333333333331;\n"
         "r_8_=s1[i_]+2;\n"
         "if(s1[i_]==3){\n"
         "   r_7_=2*r_8_;\n"