Replace div-spoiler hack with simpler code

This comes from a comment in GH issue #1027. Andy wrote the code,
Rich made the PR.
Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2253)

ssl/record/ssl3_record.c

@@ -1309,13 +1309,13 @@ void ssl3_cbc_copy_mac(unsigned char *out,
      */
     size_t mac_end = rec->length;
     size_t mac_start = mac_end - md_size;
+    size_t in_mac;
     /*
      * scan_start contains the number of bytes that we can ignore because the
      * MAC's position can only vary by 255 bytes.
      */
     size_t scan_start = 0;
     size_t i, j;
-    size_t div_spoiler;
     size_t rotate_offset;
 
     OPENSSL_assert(rec->orig_len >= md_size);
@@ -1328,24 +1328,19 @@ void ssl3_cbc_copy_mac(unsigned char *out,
     /* This information is public so it's safe to branch based on it. */
     if (rec->orig_len > md_size + 255 + 1)
         scan_start = rec->orig_len - (md_size + 255 + 1);
-    /*
-     * div_spoiler contains a multiple of md_size that is used to cause the
-     * modulo operation to be constant time. Without this, the time varies
-     * based on the amount of padding when running on Intel chips at least.
-     * The aim of right-shifting md_size is so that the compiler doesn't
-     * figure out that it can remove div_spoiler as that would require it to
-     * prove that md_size is always even, which I hope is beyond it.
-     */
-    div_spoiler = md_size >> 1;
-    div_spoiler <<= (sizeof(div_spoiler) - 1) * 8;
-    rotate_offset = (div_spoiler + mac_start - scan_start) % md_size;
 
+    in_mac = 0;
+    rotate_offset = 0;
     memset(rotated_mac, 0, md_size);
     for (i = scan_start, j = 0; i < rec->orig_len; i++) {
-        unsigned char mac_started = constant_time_ge_8_s(i, mac_start);
-        unsigned char mac_ended = constant_time_ge_8_s(i, mac_end);
+        size_t mac_started = constant_time_eq_s(i, mac_start);
+        size_t mac_ended = constant_time_lt_s(i, mac_end);
         unsigned char b = rec->data[i];
-        rotated_mac[j++] |= b & mac_started & ~mac_ended;
+
+        in_mac |= mac_started;
+        in_mac &= mac_ended;
+        rotate_offset |= j & mac_started;
+        rotated_mac[j++] |= b & in_mac;
         j &= constant_time_lt_s(j, md_size);
     }