Use different bit tricks to extend the LSB.

C gets grumpy when you shift into a sign bit. Replace it with a different bit
trick.

BUG=chromium:603502

Change-Id: Ia4cc2e2d68675528b7c0155882ff4d6230df482b
Reviewed-on: https://boringssl-review.googlesource.com/7740
Reviewed-by: Adam Langley <agl@google.com>

crypto/ec/p256-64.c

diff --git a/crypto/ec/p256-64.c b/crypto/ec/p256-64.c
index 84b6597..c4259b6 100644
--- a/crypto/ec/p256-64.c
+++ b/crypto/ec/p256-64.c
@@ -328,8 +328,7 @@
    * conditionally subtract kPrime if tmp[3] is large enough. */
   high = tmp[3] >> 64;
   /* As tmp[3] < 2^65, high is either 1 or 0 */
-  high <<= 63;
-  high >>= 63;
+  high = ~(high - 1);
   /* high is:
    *   all ones   if the high word of tmp[3] is 1
    *   all zeros  if the high word of tmp[3] if 0 */