tccgen: fix void expression side effect regression

From 7e01b20362

reduced from an app that builds/runs with gcc or clang.

tccgen.c

@@ -3278,7 +3278,6 @@ again:
         dbt_bt = dbt & VT_BTYPE;
         sbt_bt = sbt & VT_BTYPE;
         if (dbt_bt == VT_VOID) {
-            vtop->r = vtop->r2 = VT_CONST;
             goto done;
         }
         if (sbt_bt == VT_VOID) {

tests/tests2/143_void_expr.c

@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+static void f(int x)
+{
+     printf("f(%d)\n", x);
+}
+
+int main(void)
+{
+     int count = 0, i = 0;
+     for (; i < 3; ++i) {
+         printf("%d\n", i);
+         (void)(i || (f(i), ++count));
+     }
+     printf("count %d\n", count);
+     return count == 1 ? 0 : 1;
+}

tests/tests2/143_void_expr.expect

@@ -0,0 +1,5 @@
+0
+f(0)
+1
+2
+count 1