fix UB in fold.c

Author: Hugobros3

src/shady/fold.c

@@ -172,9 +172,11 @@ break;
             }
             case convert_op: {
                 const Type* dst_t = first(payload.type_arguments);
-                uint64_t bitmask = ~(UINT64_MAX << get_type_bitwidth(dst_t));
+                uint64_t bitmask = 0;
                 if (get_type_bitwidth(dst_t) == 64)
                     bitmask = UINT64_MAX;
+                else
+                    bitmask = ~(UINT64_MAX << get_type_bitwidth(dst_t));
                 if (dst_t->tag == Int_TAG) {
                     if (all_int_literals) {
                         uint64_t old_value = get_int_literal_value(*int_literals[0], int_literals[0]->is_signed);